- 1. Background and Motivation
- 2. CLIC Design
- 3. CLIC Memory-mapped Registers
- 3.1. M-Mode CLIC Memory Map
- 3.2. CLIC configuration (
cliccfg) - 3.3. CLIC Information (
clicinfo) - 3.4. CLIC Interrupt Pending (
clicintip) - 3.5. CLIC Interrupt Enable (
clicintie) - 3.6. CLIC Interrupt Attribute Register (
clicintattr) - 3.7. CLIC Interrupt Input Control (
clicintctl) - 3.8. S-Mode CLIC regions for M/S/U harts
- 3.9. U-Mode CLIC regions in M/U harts or M/S/U harts
- 3.10. CLIC memory map for Multiple Harts
- 4. CLIC CSRs
- 4.1. Changes to
xstatusCSRs - 4.2. Changes to Delegation (
xedeleg/xideleg) CSRs - 4.3. Changes to
xie/xipCSRs - 4.4. New
xtvecCSR mode for CLIC - 4.5. New
xtvtCSRs - 4.6. Changes to
xepcCSRs - 4.7. Changes to
xcauseCSRs - 4.8. Next Interrupt Handler Address and Interrupt-Enable CSRs (
xnxti) - 4.9. New Interrupt Status (
xintstatus) CSRs - 4.10. New Interrupt-Level Threshold (
xintthresh) CSRs - 4.11. New CLIC Base (
mclicbase) CSR
- 4.1. Changes to
- 5. CLIC Implementation Parameters
- 6. CLIC Interrupt Operation
- 7. Interrupt Handling Software
- 8. Calling C-ABI functions as Interrupt Handlers
- 9. Interrupt-Driven C-ABI Model
- 10. Alternate Interrupt Models for Software Vectoring
- 11. Managing Interrupt Stacks Across Privilege Modes
- 12. Separating stack per interrupt level
- 13. CLIC Interrupt IDs
The Core-Local Interrupt Controller (CLIC) is designed to provide low-latency, vectored, pre-emptive interrupts for RISC-V systems. When activated the CLIC subsumes and replaces the original RISC-V basic local interrupt scheme. The CLIC design has a base design that requires minimal hardware, but supports additional extensions to provide hardware acceleration. The goal of the CLIC design is to provide support for a variety of software ABI and interrupt models, without complex hardware that can impact high-performance processor implementations.
The CLIC also supports a new Selective Hardware Vectoring feature that allow users to optimize each interrupt for either faster response or smaller code size.
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Note
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While the current CLIC design provides only hart-local interrupt control, future additions might also support directing interrupts to harts within a core, hence the name (also CLIC sounds better than HLIC or HIC). |
The existing RISC-V interrupt system already supports interrupt
preemption, but only based on privilege mode. At any point in time, a
RISC-V hart is running with a current privilege mode. The global
interrupt enable bits, MIE/SIE/UIE, held in the
mstatus/sstatus/ustatus registers respectively, control whether
interrupts can be taken for the current or higher privilege modes;
interrupts are always disabled for lower-privileged modes. Any
enabled interrupt from a higher-privilege mode will stop execution at
the current privilege mode, and enter the handler at the higher
privilege mode. Each privilege mode has its own interrupt state
registers (mepc/mcause for M-mode, sepc/scause for S-mode,
uepc/ucause for U-mode with N extension) to support preemption, or
generically xepc for privilege mode x. Preemption by a
higher-privilege-mode interrupt also pushes current privilege mode and
interrupt enable status onto the xpp and xpie
stacks in the xstatus register of the higher-privilege mode.
The xtvec register specifies both the interrupt mode and the base
address of the interrupt vector table. The low bits of the WARL
xtvec register indicate what interrupt model is supported. The
original settings of xtvec mode (*00 and *01) indicate use of the
original basic interrupt model with either non-vectored or vectored transfer to a handler
function, with the 4-byte (or greater) aligned table base address held
in the upper bits of xtvec.
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Note
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WARL means "Write Any, Read Legal" indicating that any value can be attempted to be written but only some supported values will actually be written. |
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Note
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CLIC mode is enabled using previously reserved values (*11)
in the low two bits of xtvec.
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The standard RISC-V platform-level interrupt controller (PLIC)
provides centralized interrupt prioritization and routing for shared
platform-level interrupts, and sends only a single external interrupt
signal per privilege mode (meip/seip/ueip) to each hart.
The CLIC complements the PLIC. Smaller single-core systems might have
only a CLIC, while multicore systems might have a CLIC per-core and a
single shared PLIC. The PLIC xeip signals are treated as
hart-local interrupt sources by the CLIC at each core.
The existing original basic interrupt controller was a small unit
that provided local interrupts based on earlier designs, and managed
the software, timer, and external interrupt signals
(xsip/xtip/xeip signals in
the xip register). This basic controller also allowed additional
custom fast interrupt signals to be added in bits 16 and up of the
xip register.
New settings of xtvec mode as described below are used to enable CLIC
modes instead of the original basic interrupt modes. Platform profiles may
require either or both of the original basic and CLIC interrupt modes.
This section describes the design of the Core-Local Interrupt Controller that receives interrupt signals and presents the next interrupt to be processed to the processor.
The CLIC extends interrupt preemption to support up to 256 interrupt levels for each privilege mode, where higher-numbered interrupt levels can preempt lower-numbered interrupt levels. Interrupt level 0 corresponds to regular execution outside of an interrupt handler. Levels 1—255 correspond to interrupt handler levels. Platform profiles will dictate how many interrupt levels must be supported.
Incoming interrupts with a higher interrupt level can preempt an active interrupt handler running at a lower interrupt level in the same privilege mode, provided interrupts are globally enabled in this privilege mode.
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Note
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Existing RISC-V interrupt behavior is retained, where incoming interrupts for a higher privilege mode can preempt an active interrupt handler running in a lower privilege mode, regardless of global interrupt enable in lower privilege mode. |
The CLIC subsumes the functionality of the fast local interrupts
previously provided in bits 16 and up of xip/xie, so these are no
longer visible in xip/xie.
The existing timer (mtip/stip/utip), software
(msip/ssip/usip), and external interrupt inputs
(meip/seip/ueip) are treated as additional local interrupt
sources, where the privilege mode, interrupt level, and priority can
be altered using memory-mapped clicintctl[i] registers. For
each of
meip/mtip/msip/seip/stip/ssip/ueip/utip/usip, an
8-bit configuration register is provided, which follows the format of
the above.
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Note
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In CLIC mode, interrupt delegation for these signals is achieved
via changing the interrupt’s privilege mode in the CLIC Interrupt
Attribute Register (clicintattr), as with any other CLIC
interrupt input.
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Each hart has a separate CLIC accessed by a separate address region. When a system has PMP, this region must be made accessible to the M-mode software running on the hart.
The base address of M-Mode CLIC memory-mapped registers is specified
at a new CLIC Base (mclicbase) CSR.
The CLIC memory map supports up to 4096 total interrupt inputs.
M-mode CLIC memory map
Offset
0x0000 1B RW cliccfg
0x0004 4B R clicinfo
0x1000+4*i 1B/input R or RW clicintip[i]
0x1001+4*i 1B/input RW clicintie[i]
0x1002+4*i 1B/input RW clicintattr[i]
0x1003+4*i 1B/input RW clicintctl[i]
...
0x4FFC 1B/input R or RW clicintip[4095]
0x4FFD 1B/input RW clicintie[4095]
0x4FFE 1B/input RW clicintattr[4095]
0x4FFF 1B/input RW clicintctl[4095]If an input i is not present in the hardware, the corresponding
clicintip[i], clicintie[i], clicintattr[i],
clicintctl[i] memory locations appear hardwired to zero.
The CLIC has a single memory-mapped 8-bit global configuration
register, cliccfg, that defines how many privilege modes are supported,
how the clicintctl[i] registers are subdivided into level and
priority fields, and whether selective hardware vectoring is supported.
The cliccfg register has three WARL fields, a 2-bit nmbits field,
a 4-bit nlbits field, and a 1-bit nvbits field, plus a reserved
bit WPRI-hardwired to zero in current spec.
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Note
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WPRI means "Writes Preserve Values, Reads Ignore Values" indicating whole read/write fields are reserved for future use. Software should ignore the values read from these fields, and should preserve the values held in these fields when writing values to other fields of the same register. For forward compatibility, implementations that do not furnish these fields must hardwire them to zero. |
cliccfg register layout
bits field
7 reserved (WPRI 0)
6:5 nmbits[1:0]
4:1 nlbits[3:0]
0 nvbitsThe cliccfg register resets to 0 (i.e., all interrupts are M-mode at
level 255).
Detailed explanation for each field are described in the following sections.
The 2-bit cliccfg.nmbits WARL field encodes how many bits in a
clicintattr[i].mode register are used to hold an input i's
privilege mode.
M-mode-only systems do not support privilege-mode fields in the
clicintattr.mode registers (cliccfg.nmbits = 0).
M/U-mode systems with user-level interrupts support cliccfg.nmbits =
0 or 1. If cliccfg.nmbits = 0, then all interrupts are treated as
M-mode interrupts. If the cliccfg.nmbits = 1, then a value of 1 in
the MSB of a clicintattr[i].mode register indicates that interrupt
intput is taken in M-mode, while a value of 0 indicates that interrupt
is taken in U-mode.
M/S/U-mode systems support 0, 1, or 2 bits of privilege-mode field.
cliccfg.nmbits = 0 indicates that all local interrupts are taken in
M-mode. cliccfg.nmbits = 1 indicates that the MSB selects between M-mode
(1) and S-mode (0). cliccfg.nmbits = 2 indicates that the two MSBs of
each clicintattr[i].mode register encode the interrupt’s privilege
mode using the same encoding as the mstatus.mpp field.
Encoding for RISC-V privilege levels (mstatus.mpp) Level Encoding Name Abbreviation 0 00 User/Application U 1 01 Supervisor S 2 10 Reserved 3 11 Machine M
priv-modes nmbits clicintattr[i].mode Interpretation
M 0 xx M-mode interrupt
M/U 0 xx M-mode interrupt
M/U 1 0x U-mode interrupt
M/U 1 1x M-mode interrupt
M/S/U 0 xx M-mode interrupt
M/S/U 1 0x S-mode interrupt
M/S/U 1 1x M-mode interrupt
M/S/U 2 00 U-mode interrupt
M/S/U 2 01 S-mode interrupt
M/S/U 2 10 Reserved (or extended S-mode)
M/S/U 2 11 M-mode interrupt
M/S/U 3 xx Reserved
The 4-bit cliccfg.nlbits WARL field indicates how many bits
immediately below the cliccfg.nmbits privilege-mode bits encode the
level at which the interrupt is taken. Valid values are 0—8.
If the nlbits > CLICINTCTLBITS, then the lower bits of
the 8-bit interrupt level are assumed to be all 1s. If nlbits <
8, then the lower bits of the 8-bit interrupt level are assumed to be
all 1s. The following table shows how levels are encoded in either of
these two cases.
#bits encoding interrupt levels
1 l....... 127, 255
2 ll...... 63, 127, 191, 255
3 lll..... 31, 63, 95, 127, 159, 191, 223, 255
4 llll.... 15,31,47,63,79,95,111,127,143,159,175,191,207,223,239,255
"l" bits are available variable bits in level specification
"." bits are non-existent bits for level encoding, assumed to be 1
If nlbits = 0, then all interrupts are treated as level 255.
Examples of cliccfg settings:
CLICINTCTLBITS nlbits clicintctl[i] interrupt levels
0 2 ........ 255
1 2 l....... 127,255
2 2 ll...... 63,127,191,255
3 3 lll..... 31,63,95,127,159,191,223,255
4 1 lppp.... 127,255
"." bits are non-existent bits for level encoding, assumed to be 1 "l" bits are available variable bits in level specification "p" bits are available variable bits in priority specification
The least-significant bits in clicintctl[i] that are not
configured to be part of the mode or level are used to prioritize
among interrupts pending-and-enabled at the same privilege mode and
interrupt level. The highest-priority interrupt at a given privilege
mode and interrupt level is taken first. In case there are multiple
pending-and-enabled interrupts at the same highest priority, the
highest-numbered interrupt is taken first.
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Note
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The highest numbered interrupt wins in a tie. This is the same as the original basic interrupt mode, but different than the PLIC. |
Any implemented priority bits are treated as the most-significant bits
of a 8-bit unsigned integer with lower unimplemented bits set to 1.
For example, with one priority bit (p111_1111), interrupts can be
set to have priorities 127 or 255, and with two priority bits
(pp11_1111), interrupts can be set to have priorities 63, 127, 191,
or 255.
The single-bit read-only nvbits field in cliccfg specifies whether
the selective interrupt hardware vectoring feature is implemented or not.
This selective hardware vectoring feature gives users the flexibility to select the behavior for each interrupt: either hardware vectoring or non-vectoring. As a result, it allows users to optimize each interrupt and enjoy the benefits of both behaviors. More specifically, hardware vectoring has the advantage of faster interrupt response at the price of slightly increasing the code size (to save/restore contexts). On the other hand, non-vectoring has the advantage of smaller code size (by sharing and reusing one copy of common code to save/restore contexts) at the price of slightly slower interrupt response.
If nvbits = 0, then selective interrupt hardware vectoring is not implemented.
So all interrupts are non-vectored and are directed to the common code
at xtvec register.
If nvbits = 1, then selective interrupt hardware vectoring is implemented.
The bit clicintattr[i].shv controls the vectoring behavior of
interrupt i. If clicintattr[i].shv is 0, then
the interrupt is non-vectored and always jumps to the common code at
xtvec.
If clicintattr[i].shv is 1, then the interrupt is hardware vectored
to the trap-handler function pointer specified in xtvt CSR.
This allows some interrupts to
all jump to a common base address held in xtvec, while the others are
vectored in hardware via a table pointed to by the additional xtvt
CSR.
This is a read-only register to show information useful for debugging.
clicinfo register layout
bits field
31:25 reserved (WPRI 0)
24:21 CLICINTCTLBITS
20:13 version (for version control)
20:17 for architecture version, 16:13 for implementation version
12:0 num_interrupt (number of maximum interrupt inputs supported)The num_interrupt field specifies the actual number of maximum interrupt
inputs supported in this implementation.
The version field specifies the implementation version of CLIC. The upper
4-bit specifies the architecture version, and the lower 4-bit specifies
the implementation version.
The CLICINTCTLBITS field specifies how many hardware bits are actually
implemented in the clicintctl registers, with 0 ≤ CLICINTCTLBITS ≤ 8.
The implemented bits are kept left-justified in the most-significant bits of
each 8-bit clicintctl[i] register, with the lower unimplemented bits
treated as hardwired to 1.
Each interrupt input has a dedicated interrupt pending bit (clicintip[i])
and occupies one byte in the memory map for ease of access.
clicintip[i] is a read-write register. Software-based (direct) writes
to these pending bits have priority over hardware-based writes (triggers).
For level-triggered interrupts, users should not clear the pending bits directly but instead should clear the interrupt sources (devices).
For edge-triggered interrupts, to speed up interrupt processing, hardware is designed to help clearing interrupt pending bits. Nevertheless, the clearing mechanism and timing are different for Selective Hardware Vectoring (SHV) mode and non-SHV (common code) mode. Detailed operations for each case are described below.
When a Selective Hardware Vectoring (SHV or vectored) interrupt is selected and serviced, the hardware will automatically clear the corresponding pending bit in edge-triggered mode. In this case, software does not need to clear pending bit at all in the service routine.
In contrast, when non-SHV (common code) interrupt is selected, the hardware
will not automatically clear the pending bit in edge-triggered mode. Instead,
hardware will clear the corresponding pending bit only when software uses a
csrrsi/csrrci mnxti instruction to select this interrupt and return its
entry address. However, if the CSR instruction does not include write side effects
(e.g., csrr t0, mnxti), then no state update on any CSR occurs and thus the
interrupt pending bit is not cleared. This behavior allows software to optimize the
selection and execution of interrupts using mnxti.
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Note
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During normal operation, software does not need to clear pending bits because CLIC hardware already supports automatic clearing of pending bits for edge-triggered interrupts. As for level-triggered interrupts, they should be cleared at interrupt sources (devices) so no need to clear the pending bits. Therefore, software usually only needs to modify pending bits in the initialization process or testing. |
This is because our CLIC HW already supports automatic clearing of pending bits for edge-triggered interrupts and thus SW never has to worry about clearing pending bits regardless of interrupt types (level or edge).
Each interrupt input has a dedicated interrupt-enable bit (clicintie[i])
and occupies one byte in the memory map for ease of access. This control bit is
read-write to enable/disable the corresponding interrupt.
This is a WARL read-write register to specify various attributes for each interrupt.
Bits Field
7:6 mode
5:3 reserved (WPRI 0)
2:1 trig
0 shvThe 1-bit shv field is used for Selective Hardware Vectoring.
If shv is 0, it assigns this interrupt to be non-vectored and thus it jumps
to the common code at xtvec.
If shv is 1, it assigns this interrupt to be hardware vectored and thus it
automatically jumps to the trap-handler function pointer specified in xtvt CSR.
This feature allows some interrupts to all jump to a common base address held
in xtvec, while the others are vectored in hardware via a table pointed to
by the additional xtvt CSR.
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Note
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if cliccfg.nvbits is 0, the selective interrupt hardware vectoring
feature is not implemented and thus shv field appears hardwired to
zero (WARL 0).
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The 2-bit trig WARL field specifies the trigger type and polarity for each
interrupt input. Bit 1, trig[0], is defined as "edge-triggered"
(0: level-triggered, 1: edge-triggered); while bit 2, trig[1], is defined
as "negative-edge" (0: positive-edge, 1: negative-edge).
More specifically, there can be four possible combinations:
positive level-triggered, negative level-triggered, positive edge-triggered,
and negative edge-triggered.
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Note
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Some implementations may want to save these bits so only certain trigger types are supported. In this case, these bits become hard-wired to fixed values (WARL). |
The 2-bit mode WARL field specifies which privilege mode this interrupt
operates in. This field uses the same encoding as the mstatus.mpp
(11: machine mode, 01: supervisor mode, 00 user mode). The default value
for clicintattr.mode is 11 to represent machine mode. The valid length of
this field can be programmed with cliccfg.nmbits.
The CLIC design supports up to 4096 interrupt inputs per hart, with
each interrupt input i having an 8-bit memory-mapped WARL
control register, clicintctl[i]. The first 16 interrupt
inputs are reserved for the original basic mode interrupts present in
the low 16 bits of the xip and xie registers, so up to 4080 local
external interrupts can be added.
A fixed parameter of the CLIC (CLICINTCTLBITS) is how many total bits
are present in the clicintctl registers , with 0 ≤
CLICINTCTLBITS ≤ 8. The implemented bits are kept left-justified
in the most-significant bits of each 8-bit clicintctl[i]
register, with the lower unimplemented bits treated as hardwired to 1.
These configuration bits are interpreted as level and
priority depend on the setting of the cliccfg register as
described below.
Each interrupt input also has an orthogonal interrupt-enable bit
(clicintie[i]) as well as an interrupt-pending bit (clicintip[i])
in the memory map.
To select an interrupt to present to the core, the CLIC hardware
combines the valid bits in clicintattr.mode and
clicintctl to form an unsigned value, then picks the global maximum
across all pending-and-enabled interrupts based on this value.
Next, the cliccfg setting determines how to split
the maximum clicintctl value into interrupt level and interrupt
priority. Finally, the interrupt level of the selected interrupt is
compared with the interrupt-level threshold of the associated privelege
mode to determine whether it is qualified or masked by the threshold
(and thus no interrupt is presented).
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Warning
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Selecting an interrupt at a high privilege mode masks any interrupt at a lower privilege mode since the higher-privilege mode causes the interrupt signal to appear more urgent than any lower-privilege mode interrupt. |
The 4096 CLIC interrupt vectors are given unique identification numbers
with xcause Exception Code (exccode) values. To maintain backward
compatibility, the original basic mode interrupts retain their original
cause values, while the new interrupts are numbered starting at 16.
To support Non-Maskable Interrupt (NMI), the exccode for NMI is defined as 0xFFF with mcause.Interrupt=0.
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Note
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When upgrading an earlier original basic interrupt system design that had local interrupts attached directly to bits 16 and above, these local interrupts can be now attached as CLIC inputs 16 and above to retain the same interrupt IDs. |
Supervisor-mode CLIC regions only expose interrupts that have been configured to be supervisor-accessible via the M-mode CLIC region. System software must configure virtual memory and PMP permissions to only allow access to this region from appropriate supervisor-mode code.
Layout of Supervisor-mode CLIC regions
0x000+4*i 1B/input R or RW clicintip[i]
0x001+4*i 1B/input RW clicintie[i]
0x002+4*i 1B/input RW clicintattr[i]
0x003+4*i 1B/input RW clicintctl[i]Any interrupt i that is not accessible to S-mode appears as
hard-wired zeros in clicintip[i], clicintie[i], clicintattr[i], and
clicintctl[i].
Where cliccfg.nmbits = 0, all interrupts are M-mode only, and all
are inaccessible to S-mode.
Where cliccfg.nmbits = 1, if clicintattr[i].mode is set to S-mode
(bit 7 is clear), interrupt i is visible in the S-mode region.
Where cliccfg.nmbits = 2, if bit 7 of clicintattr[i].mode is clear
(S-mode or U-mode), interrupt i is visible through the S-mode region
This allows the supervisor region to be
used to selectively configure the interrupt as S-mode or U-mode.
User-mode CLIC regions only expose interrupts that have been configured to be user-accessible via the M-mode CLIC region. System software must configure virtual memory and PMP permissions to only allow access to this region from appropriate user-mode code.
Layout of user-mode CLIC regions
0x000+4*i 1B/input R or RW clicintip[i]
0x001+4*i 1B/input RW clicintie[i]
0x002+4*i 1B/input RW clicintattr[i]
0x003+4*i 1B/input RW clicintctl[i]Any interrupt i that is not accessible to U-mode appears as
hard-wired zeros in clicintip[i], clicintie[i], clicintattr[i], and
clicintctl[i].
Where cliccfg.nmbits = 0, all interrupts are M-mode only, and all
are inaccessible to U-mode.
In M/U-only harts, where cliccfg.nmbits = 1, if clicintattr[i].mode
is set to U-mode (bit 7 is clear), then interrupt i is visible in the
U-mode region.
In M/S/U harts, if cliccfg.nmbits < 2 then all interrupts are
either M-mode or S-mode, and all are inaccessible to U-mode.
In M/S/U harts, where cliccfg.nmbits = 2, if clicintattr[i].mode is
set to U-mode (bits 6 and 7 are clear), then interrupt i is visible
in the U-mode region.
This section describes the CLIC-related hart-specific CSRs. When in original basic interrupt mode, the behavior is intended to be software compatible with basic-mode-only systems.
The interrupt-handling CSRs are listed below, with changes and additions for CLIC mode described in the following sections.
Number Name Description
0xm00 xstatus Status register
0xm02 xedeleg Exception delegation register
0xm03 xideleg Interrupt delegation register (INACTIVE IN CLIC MODE)
0xm04 xie Interrupt-enable register (INACTIVE IN CLIC MODE)
0xm05 xtvec Trap-handler base address / interrupt mode
(NEW) 0xm07 xtvt Trap-handler vector table base address
0xm40 xscratch Scratch register for trap handlers
0xm41 xepc Exception program counter
0xm42 xcause Cause of trap
0xm43 xtval Bad address or instruction
0xm44 xip Interrupt-pending register (INACTIVE IN CLIC MODE)
(NEW) 0xm45 xnxti Interrupt handler address and enable modifier
(NEW) 0xm?? xintstatus Current interrupt levels
(NEW) 0xm48 xscratchcsw Conditional scratch swap on priv mode change
(NEW) 0xm49 xscratchcswl Conditional scratch swap on level change
(NEW) 0xm?? xintthresh Interrupt-level threshold
(NEW) 0x3?? mclicbase Base address for CLIC memory mapped registers
m is the nibble encoding the privilege mode (M=0x3, S=0x1, U=0x0)When in original basic interrupt mode, the xstatus register behavior is unchanged
(i.e., backwards-compatible with original basic mode). When in CLIC mode,
the xpp and xpie in xstatus are now accessible
via fields in the xcause register.
In CLIC mode, the CLIC input configuration clcintcfg[i]
specifies the privilege mode in which each interrupt should be taken,
so the xideleg CSR ceases to have effect in CLIC mode. The xideleg
CSR is still accessible and state bits retain their values when
switching between CLIC and original basic interrupt modes.
Exception delegation specified by xedeleg functions the same in CLIC
mode as in original basic mode.
The xie CSR appears hardwired to zero in CLIC mode, replaced by separate
memory-mapped interrupt enables (clicintie[i]).
The xip CSR appears hardwired to zero in CLIC mode, replaced by
separate memory-mapped interrupt pendings (clicintip[i]).
In systems that support both original basic and CLIC modes, the state bits in
xie and xip retain their value when switching between modes.
The new CLIC interrupt-handling mode is encoded as a new state in the
existing xtvec WARL register, where the low two bits of xtvec are
11. In this mode, the trap vector base address held in
xtvec is constrained to be aligned on a 64-byte or larger
power-of-two boundary.
mtvec Action on Interrupt
aaaa00 pc := OBASE (original non-vectored basic mode)
aaaa01 pc := OBASE + 4 * exccode (original vectored basic mode)
000011 (CLIC mode)
(non-vectored)
pc := NBASE if clicintattr[i].shv = 0
|| if cliccfg.nvbits = 0
(vector not supported)
(vectored)
pc := M[TBASE + XLEN/8 * exccode)] & ~1 if clicintattr[i].shv = 1
000010 Reserved
xxxx1? (xxxx!=0000) Reserved
OBASE = mtvec[XLEN-1:2]<<2 # Original vector base was at least 4-byte aligned.
NBASE = mtvec[XLEN-1:6]<<6 # New vector base is at least 64-byte aligned.
TBASE = mtvt # Trap vector table base is aligned proportionally
# to the maximum number of interrupts (see details
# below).
In CLIC mode, writing 0 to clicintattr[i].shv
sets interrupt i to non-vectored,
where the processor jumps to the
trap handler address held in the upper XLEN-6 bits of
xtvec for all exceptions and interrupts in privilege mode
x. Similarly, if the selective hardware
vectoring feature is not implemented (cliccfg.nvbits is 0),
all interrupts are non-vectored and behave the same.
On the other hand, writing 1 to clicintattr[i].shv
sets interrupt i to vectored. In this case, the processor
switches to the handler’s privilege mode and sets the hardware
vectoring bit xinhv in xcause, then fetches an XLEN-bit handler
address from the in-memory table whose base address (TBASE) is in
xtvt. The trap handler function address is fetched from
TBASE+XLEN/8*exccode. If the fetch is successful, the processor
clears the low bit of the handler address, sets the PC to this handler
address, then clears the xinhv bit in xcause. The overall effect
is:
pc := M[TBASE + XLEN/8 * exccode] & ~1
# Vector table layout for RV32 (4-byte function pointers)
mtvt -> 0x800000 # Interrupt 0 handler function pointer
0x800004 # Interrupt 1 handler function pointer
0x800008 # Interrupt 2 handler function pointer
0x80000c # Interrupt 3 handler function pointer
# Vector table layout for RV64 (8-byte function pointers)
mtvt -> 0x800000 # Interrupt 0 handler function pointer
0x800008 # Interrupt 1 handler function pointer
0x800010 # Interrupt 2 handler function pointer
0x800018 # Interrupt 3 handler function pointer|
Note
|
The original basic vectored mode simply jumped to an address in the trap vector table, while the new CLIC vectored mode reads a handler function address from the table, and jumps to it in hardware. |
|
Note
|
The vector table contains vector addresses rather than instructions because it simplifies static initialization in C. More specifically, the entries in the table are simple XLEN-bit function pointers. |
|
Note
|
The hardware vectoring bit xinhv is provided to allow resumable
traps on fetches to the trap vector table.
|
Implementations might support only one of original basic or CLIC mode.
If only basic mode is supported, writes to bit 1 are ignored and it is
always set to zero (current behavior). If only CLIC mode is supported,
writes to bit 1 are also ignored and it is always set to one. CLIC
mode hardwires xtvec bits 2-5 to zero (assuming no further CLIC
extensions are supported).
For permissions-checking purposes, the memory access to retrieve the
function pointer for vectoring is treated as a load with the privilege
mode of the interrupt handler. If there is an access exception on the
table load, xepc holds the faulting address. If this was a page
fault, the table load can be resumed by returning with xepc pointing
to the table entry and the trap handler mode bit set.
Instruction fetch at the handler address might cause misaligned or
access exceptions, which are reported with xepc containing the
faulting instruction fetch address.
In CLIC mode, synchronous exception traps always jump to NBASE.
The xtvt WARL XLEN-bit CSR holds the base address of the trap vector
table, aligned on a 64-byte or greater power-of-two boundary. The actual
alignment can be determined by writing ones to the low-order bits then reading
them back. Values other than 0 in the low 6 bits of xtvt are reserved.
In systems that support both original basic and CLIC modes, the xtvt CSR is
still accessible in basic mode (but does not have any effect).
The xepc CSRs behave the same in both modes, capturing the PC at
which execution was interrupted.
In both original basic and CLIC modes, the xcause CSR is written at the
time an interrupt or synchronous trap is taken, recording the reason for
the interrupt or trap. For CLIC mode, xcause is also extended to record
more information about the interrupted context, which is used to
reduce the overhead to save and restore that context for an xret
instruction. CLIC mode xcause also adds state to record progress
through the trap handling process.
mcause Bits Field Description XLEN-1 Interrupt Interrupt=1, Exception=0 30 minhv Hardware vectoring in progress when set 29:28 mpp[1:0] Previous privilege mode, same as mstatus.mpp 27 mpie Previous interrupt enable, same as mstatus.mpie 26:24 (reserved) 23:16 mpil[7:0] Previous interrupt level 15:12 (reserved) 11:0 Exccode[11:0] Exception/interrupt code
The mcause.mpp and mcause.mpie fields mirror the mstatus.mpp and
mstatus.mpie fields, and are aliased into mcause to reduce context
save/restore code.
If the hart is currently running at some privilege mode (pp) at some
interrupt level (pil) and an enabled interrupt becomes pending at
any interrupt level in a higher privilege mode or if an interrupt at a
higher interrupt level in the current privilege mode becomes pending
and interrupts are globally enabled in this privilege mode, then
execution is immediately transferred to a handler running with the new
interrupt’s privilege mode (x) and interrupt level (il).
The CSR xepc is set to the PC of the interrupted application
code or preempted interrupt handler, while the xcause
register now captures the previous privilege mode (pp), interrupt
level (pil) and interrupt enable (pie), as well as the id of the
interrupt in exccode.
In systems supporting both original basic and CLIC modes, the new
CLIC-specific fields (minhv, mpp, mpil, mpie) appear to be
hardwired to zero in basic mode for backwards compatibilty. When
basic mode is written to xtvec, the new xcause state fields
(mhinv and mpil) are zeroed. The other new xcause fields,
mpp and mpie, appear as zero in the xcause CSR but the corresponding
state bits in the mstatus register are not cleared.
The supervisor scause register has only a single spp bit (to
indicate user/supervisor) mirrored from sstatus.spp, while the user
ucause register has no upp bit as interrupts can only have come
from user mode.
scause
Bits Field Description
XLEN-1 Interrupt Interrupt=1, Exception=0
30 sinhv Hardware vectoring in progress when set
29 (reserved)
28 spp Previous privilege mode, same as sstatus.spp
27 spie Previous interrupt enable, same as sstatus.spie
26:24 (reserved)
23:16 spil[7:0] Previous interrupt level
15:12 (reserved)
11:0 exccode[11:0] Exception/interrupt code
ucause
Bits Field Description
XLEN-1 Interrupt Interrupt=1, Exception=0
30 uinhv Hardware vectoring in progress when set
29:28 (reserved)
27 upie Previous interrupt enable, same as ustatus.upie
26:24 (reserved)
23:16 upil[7:0] Previous interrupt level
15:12 (reserved)
11:0 exccode[11:0] Exception/interrupt code
The xnxti CSR can be used by software to service the next horizontal
interrupt for the same privilege mode when it has greater level than
the saved interrupt context (held in xcause`.pil`), without incuring
the full cost of an interrupt pipeline flush and context save/restore.
The xnxti CSR is designed to be accessed using CSRRSI/CSRRCI
instructions, where the value read is a pointer to an entry in the
trap handler table and the write back updates the interrupt-enable
status. In addition, accesses to the xnxti have side-effects that
update the interrupt context state.
|
Note
|
This is different than a regular CSR instruction as the value returned is different from the value used in the read-modify-write operation. |
A read of the xnxti CSR returns either zero, indicating there is no
suitable interrupt to service or that the highest ranked interrupt is
SHV or that the system is not in a CLIC mode, or returns a non-zero
address of the entry in the trap handler table for software trap
vectoring.
|
Note
|
The xtvt CSR could be set to memory addresses such that a table
entry was at address zero, and this would be indistinguishable from
the no-interrupt case.
|
If the CSR instruction that acccesses xnxti includes a write, the
xstatus CSR is the one used for the read-modify-write portion of the
operation, while the xcause register’s exccode field and the
xintstatus register’s xil field can also be updated with
the new interrupt id and level respectively.
|
Note
|
Following the usual convention for CSR instructions, if the CSR
instruction does not include write side effects (e.g., csrr t0,
mnxti), then no state update on any CSR occurs. This can be used to
determine if an interrupt could be taken without actually updating
xil and exccode.
|
The xnxti CSR is intended to be used inside an interrupt handler
after an initial interrupt has been taken and xcause and xepc
registers updated with the interrupted context and the id of the
interrupt.
// Pseudo-code for csrrsi rd, mnxti, uimm[4:0] in M mode.
mstatus |= uimm[4:0]; // Performed regardless of interrupt readiness.
if (clic.priv==M && clic.level > mcause.pil
&& (cliccfg.nvbits==0 || clicintattr.shv==0) ) {
// The CLIC interrupt should be serviced before returning to the saved context,
// unless it's a selectively hardware vectored interupt.
minstatus.mil = clic.level; // Update hart's interrupt level.
mcause.exccode = clic.id; // Update interrupt id.
rd = TBASE + XLEN/8 * clic.id; // Return pointer to trap handler entry.
} else {
// No interrupt, or a selectively hardware vectored interrupt, or in non-CLIC mode.
rd = 0;
}|
Note
|
Vertical interrupts to different privilege modes will be taken
preemptively by the hardware, so xnxti effectively only ever handles
the next interrupt in the same privilege mode.
|
In original basic mode, reads of xnxti return 0, updates to xstatus proceed
as in CLIC mode, but updates to xintstatus and xcause do not take
effect.
A new M-mode CSR, mintstatus, holds the active interrupt level for
each supported privilege mode. These fields are read-only. The
primary reason to expose these fields is to support debug.
mintstatus fields 31:24 mil 23:16 (reserved) # To follow pattern of others. 15: 8 sil 7: 0 uil
Corresponding supervisor mode, sintstatus, and user, uintstatus,
provide restricted views of mintstatus.
sintstatus fields 31:16 (reserved) 15: 8 sil 7: 0 uil
uintstatus fields 31: 8 (reserved) 7: 0 uil
The xintstatus registers are accessible in original basic mode for system that
support both modes.
The interrupt-level threshold (xintthresh) is a new read-write CSR,
which holds an 8-bit field (th) for the threshold level of the
associated privilege mode.
A typical usage of the interrupt-level threshold is for implementing critical sections. The current handler can temporarily raise its effective interrupt level to implement a critical section among a subset of levels, while still allowing higher interrupt levels to preempt.
The current hart’s effective interrupt level would then be:
effective_level = max( xintstatus.mil, xintthresh.th )
The max is used to prevent a hart from dropping below its original level which would break assumptions in design, and also makes it simple for software to remove threshold without knowing its own level by simply writing zero.
The interrupt-level threshold is only valid when running in associated privilege mode and not in other modes. This is because interrupts for lower privilege modes are always disabled, whereas interrupts for higher privilege modes are always enabled. For example, machine-mode interrupts will not be masked by machine-mode threshold setting when running in user mode. This is analogous to how mstatus.mie does not mask machine-mode interrupts when running in lower privilege modes.
|
Note
|
This behavior significantly reduces the hardware cost because it only needs to select one global maximum interrupt and compare with the threshold of the associated mode (while ignoring thresholds in other modes). Otherwise, hardware would have to select multiple maximum interrupts (one per mode), compare and qualify with their associated thresholds, then pick a qualified maximum interrupt with the highest privilege mode. |
The machine mode mclicbase CSR is an XLEN-bit read-only register
providing the base address of CLIC memory mapped registers.
Its value should be configured or set up at the platform level to indicate
the starting address of CLIC memory mapped registers.
Since the CLIC memory map must be aligned at a 4KiB boundary, the mclicbase
CSR has its 12 least-significant bits hardwired to zero. It is used
to inform software about the location of CLIC memory mappped registers.
Name Value Range Description
CLICANDBASIC 0-1 Implements original basic mode also?
CLICPRIVMODES 1-3 Number privilege modes: 1=M, 2=M/U, 3=M/S/U
CLICLEVELS 2-256 Number of interrupt levels including 0
NUM_INTERRUPT 4-4096 Always has MSIP, MTIP, MEIP, CSIP
CLICMAXID 12-4095 Largest interrupt ID
CLICINTCTLBITS 2-8 Number of bits implemented in clicintctl[i]
CLICCFGMBITS 0-ceil(lg2(CLICPRIVMODES)) Number of bits implemented for cliccfg.nmbits
CLICCFGLBITS 0-ceil(lg2((lg2(CLICLEVELS)))) Number of bits implemented for cliccfg.nlbits
CLICSELHVEC 0-1 Selective hardware vectoring supported?
CLICMTVECALIGN 6-13 Number hardwired zero LSBs in mtvec address.
CLICMNXTI 0-1 Has mnxti CSR implemented?
CLICMCSW 0-1 Has mscratchcsw/mscratchcswl implemented?This section describes the operation of CLIC interrupts.
At any time, a hart is running in some privilege mode with some
interrupt level. The hart’s privilege mode is held internally in the
processor but is not visible to software running on a hart (to avoid
virtualization holes), but the current interrupt level is made visible
in the xintstatus register. Interrupt level 0 corresponds to regular
execution outside of an interrupt handler.
Within a privilege mode x, if the associated global
interrupt-enable xie is clear, then no interrupts will be taken in
that privilege mode, but a pending-enabled interrupt in a higher
privilege mode will preempt current execution. If xie is set, then
pending-enabled interrupts at a higher interrupt level in the same
privilege mode will preempt current execution and run the interrupt
handler for the higher interrupt level.
As with the existing RISC-V mechanism, when an interrupt or synchronous exception is taken, the privilege mode and interrupt level are modified to reflect the new privilege mode and interrupt level. The global interrupt-enable bit of the handler’s privilege mode is cleared, to prevent preemption by higher-level interrupts in the same privilege mode.
The overall behavior is summarized in the following table: the Current
p/ie/il fields represent the current privilege mode P (not
software visible), interrupt enable in xstatus ie and interrupt
level L in xintstatus; the CLIC priv,level, and id fields
represent the highest-ranked interrupt currently present in the CLIC
with nP representing the new privilege mode, nL representing the
new interrupt level, and id representing the interrupt’s id;
Current' shows the p/ie/il context in the handler’s privilege mode;
pc represents the program counter with V representing the result
of any hardware vectoring; cde represents the xcause exccode
field; while the Previous pp/il/ie/epc columns represent previous
context fields in xcause and xepc.
Current | CLIC |-> Current' Previous p/ie/il | priv level id |-> p/ie/il pc cde pp/il/ie epc P ? ? | nP<P ? ? |-> - - - - - - - - - # Interrupt ignored P 0 ? | nP=P ? ? |-> - - - - - - - - - # Interrupts disabled P 1 ? | nP=P 0 ? |-> - - - - - - - - - # No interrupt P 1 L | nP=P 0<nL<=L ? |-> - - - - - - - - - # Interrupt ignored P 1 L | nP=P L<nL id |-> P 0 nL V id P L 1 pc # Horizontal interrupt taken P ? ? | nP>P 0 ? |-> - - - - - - - - - # No interrupt P e L | nP>P 0<nL id |-> nP 0 nL V id P L e pc # Vertical interrupt taken
To implement a critical section between interrupt handlers at
different levels in the same privilege mode, an interrupt handler at
any interrupt level can temporarily raise the interrupt-level threshold
(mintthresh.th) to mask a subset of levels,
while still allowing higher interrupt levels to preempt.
Alternatively, although not recommended due to worse system impacts, it can
clear the mode’s global interrupt-enable bit
(xie) to prevent any interrupts with the same privilege mode from
being taken.
Horizontal synchronous exception traps, which stay within a privilege mode, are serviced with the same interrupt level as the instruction that raised the exception.
Vertical synchronous exception traps, which are serviced at a higher privilege mode, are taken at interrupt level 0 in the higher privilege mode.
|
Warning
|
Traps should be avoided at any time when xepc/xcause are live
because these CSRs will be overwritten. Software should try to back them
up if needed.
|
The regular xret instructions are used to return from handlers in
privilege mode x. Execution continues at the saved privilege
mode xcause.xpp, at PC xepc, with interrupt level
xcause.xpil, and with the global interrupt enable
for the restored mode as xcause.xpie.
The xret instruction does not modify the
xcause.xpil field in xcause. The
xcause.xpp and xcause.xpie fields
are modified following the behavior previously defined for
xstatus.xpp and xstatus.xpie
respectively.
The CLIC supports multiple nested interrupt handlers, and each handler
requires some working registers. To make registers available, each
handler typically saves and restores registers from the interrupted
context on a memory-resident stack. In addition, the memory-resident
stack is used to hold other interrupted context information, such as
xepc and xcause, which are required by the xret instruction.
The standard RISC-V ABI convention is that stacks grow downwards, and that memory addresses below the current stack pointer can be dynamically altered by another agent, such as an interrupt handler.
When interrupts are taken horizontally within the same privilege mode, the interrupt handler may be able to use the same stack as the interrupted thread, by allocating a new stack frame below the current stack pointer.
When interrupts are taken vertically into a higher privilege mode, the
stack pointer must be swapped to a stack within the higher privilege
mode to avoid a security hole. The xscratch registers can be used to
hold the stack pointer of a higher-privilege mode while
lower-privilege code is executing, or xscratch can be used to point
to more extensive thread-local context that might contain a stack
pointer.
Inline interrupt handlers are small leaf functions that handle simple interrupts. To provide easy C coding for inline interrupt handlers, while reducing register save/restore overhead, we use standard interrupt attributes, which have the following syntax:
/* Small ISR to poke device to clear interrupt and increment in-memory counter. */
void __attribute__ ((interrupt))
foo (void)
{
extern volatile int INTERRUPT_FLAG;
INTERRUPT_FLAG = 0;
extern volatile int COUNTER;
#ifdef __riscv_atomic
__atomic_fetch_add (&COUNTER, 1, __ATOMIC_RELAXED);
#else
COUNTER++;
#endif
}The attribute tells the C compiler to use callee-save for all registers, so the handler has to "pay as it goes" to use registers, and only save the full caller-save set if it makes a nested regular C call. The attribute also tells the C compiler to align the function entry point on an 8-byte boundary.
.align 3
# Inline non-preemptible interrupt handler.
# Only safe for horizontal interrupts.
foo:
addi sp, sp, -FRAMESIZE # Create a frame on stack.
sw a0, OFFSET(sp) # Save working register.
sw x0, INTERRUPT_FLAG, a0 # Clear interrupt flag.
sw a1, OFFSET(sp) # Save working register.
la a0, COUNTER # Get counter address.
li a1, 1
amoadd.w x0, (a0), a1 # Increment counter in memory.
lw a1, OFFSET(sp) # Restore registers.
lw a0, OFFSET(sp)
addi sp, sp, FRAMESIZE # Free stack frame.
mret # Return from handler using saved mepc.With hardware vectoring, inline interrupt handlers can provide very rapid response for small tasks.
|
Note
|
The above entire handler executes in 13 instructions. The
INTERRUPT_FLAG store and the la require two instructions each to
build up a global address. A simple pipeline would encounter two
pipeline flushes (on entry and on exit), plus the cycles taken to fetch
the hardware vector entry.
|
These inline handlers can be used with the original basic mode design as well as the new CLIC design.
To take advantage of hardware preemption in the new CLIC design,
inline handlers must save and restore xepc and xcause before
enabling interrupts:
.align 3
# Inline preemptible interuppt handler.
# Only safe for horizontal interrupts.
foo:
#----- Interrupts disabled on entry ---#
addi sp, sp, -FRAMESIZE # Create a frame on stack.
sw a0, OFFSET(sp) # Save working register.
csrr a0, mcause # Read cause.
sw a1, OFFSET(sp) # Save working register.
csrr a1, mepc # Read epc.
csrrsi x0, mstatus, MIE # Enable interrupts.
#----- Interrupts enabled ---------#
sw a0, OFFSET(sp) # Save cause on stack.
sw x0, INTERRUPT_FLAG, a0 # Clear interrupt flag.
sw a1, OFFSET(sp) # Save epc on stack.
la a0, COUNTER # Get counter address.
li a1, 1
amoadd.w x0, (a0), a1 # Increment counter in memory.
lw a1, OFFSET(sp) # Restore epc
lw a0, OFFSET(sp) # Restore cause
csrrci x0, mstatus, MIE # Disable interrupts.
#----- Interrupts disabled ---------#
csrw mepc, a1 # Put epc back.
lw a1, OFFSET(sp) # Restore a1.
csrw mcause, a0 # Put cause back.
lw s0, OFFSET(sp) # Restore s0.
addi sp, sp, FRAMESIZE # Free stack frame.
mret # Return from handler.
#------------------------------------#|
Note
|
This version requires 10 more instructions, but reduces the time a preempting interrupt has to wait from a 13-instruction window to a 6-instruction window (the instruction that disables interrupts can be preempted before committing). |
|
Warning
|
This form cannot be used with the existing original basic scheme, unless the original interrupt pending signal is cleared before re-enabling interrupts. |
An alternative model is where all interrupt handler routines use the
standard C ABI. In this case, the CLIC would use no hardware
vectoring for the C ABI handlers and instead use a common software
trampoline, which uses the xnxti instruction to obtain the
trap-handler address. The code sequence below is annotated with an
explanation of its operation.
# Example Unix C ABI interrupt trampoline.
# Only safe for horizontal interrupts.
# FRAMESIZE should be defined appropriately to hold saved context with ABI-specified alignment.
# OFFSET should be replaced with individual stack frame locations.
# Register save/restore pseudo-code should be expanded to individual instructions.
irq_enter:
#----Interrupts disabled for 7 + SREGS instructions, where SREGS is number of registers saved. (1)
addi sp, sp, -FRAMESIZE # Allocate space on stack. (2)
sw a1, OFFSET(sp) # Save a1.
csrr a1, mcause # Get mcause of interrupted context.
sw a0, OFFSET(sp) # Save a0.
csrr a0, mepc # Get mepc of interrupt context.
bgez a1, handle_exc # Handle synchronous exception. (3)
sw a0, OFFSET(sp) # Save mepc.
sw a1, OFFSET(sp) # Save mcause of interrupted context.
sw a2-a7, OFFSET(sp) # Save other argument registers.
sw t0-t6, OFFSET(sp) # Save temporaries.
sw ra, OFFSET(sp) # 1 return address (5)
csrrsi a0, mnxti, MIE # Get highest current interrupt and enable interrupts.
# Will return original interrupt if no others appear. (6)
#----Interrupts enabled ----------------------- (7)
beqz a0, exit # Check if original interrupt vanished. (8)
service_loop: # 5 instructions in pending-interrupt service loop.
lw a1, (a0) # Indirect into handler vector table for function pointer. (9)
csrrsi x0, mstatus, MIE # Ensure interrupts enabled. (10)
jalr a1 # Call C ABI Routine, a0 has interrupt ID encoded. (11)
# Routine must clear down interrupt in CLIC.
csrrsi a0, mnxti, MIE # Claim any pending interrupt at level > mcause.pil (12)
bnez a0, service_loop # Loop to service any interrupt. (13)
#--- Restore ABI registers with interrupts enabled --- (14)
lw ra, OFFSET(sp) # Restore return address
lw t0-t6, OFFSET(sp) # Restore temporaries.
lw a2-a7, OFFSET(sp) # Restore other arguments.
lw a1, OFFSET(sp) # Get saved mcause,
exit: # Fast exit point.
lw a0, OFFSET(sp) # Get saved mepc.
csrrci x0, mstatus, MIE # Disable interrupts (15)
#---- Critical section with interrupts disabled -----------------------
csrw mcause, a1 # Restore previous context.
lw a1, OFFSET(sp) # Restore original a1 value.
csrw mepc, a0 # Restore previous context.
csrrci a0, mnxti, MIE # Claim highest current interrupt. (16)
bnez a0, service_loop # Go around if new interrupt.
lw a0, OFFSET(sp) # Restore original a0 value.
addi sp, sp, FRAMESIZE # Reclaim stack space.
mret # Return from interrupt.
#-----------------------------------------------------------------------
#-----------------------------------------------------------------------
handle_exc:
# ...
# Perform exception processing with interrupts disabled (4)
# ...
addi sp, sp, FRAMESIZE # Reclaim stack space.
mret # Return from exception
#-----------------------------------------------------------------------
An initial interrupt (II) causes entry to the handler with interrupts disabled, and
xepcandxcauseCSRs hold values representing the original interrupted context (OIC), including the PC inxepc, the privilege mode inxpp(visible in bothxcauseandxstatus), the interrupt level in {pil} (inxcause) and the interrupt enable state inxpie(visible in bothxcauseandxstatus). ThexcauseCSR and thexintstatusCSRs additionally hold information on the interrupt to be handled, includingexccodeinxcauseandxilinxintstatus. -
The interrupt trampoline needs sufficient space to store the OIC’s caller-save registers as well as its
epcandcausevalues, which are saved in a frame on the memory stack to support preemption. This routine is M-mode only so does not need to consider swapping stacks from other privilege modes. A simple constant bump of the stack pointerspis sufficient to provide space to store the OIC. -
The trap handler could have been entered by a synchronous exception instead of an interrupt, which can be determined by examining the sign bit of the returned
xcausevalue. If the trap was for an exception (sign bit zero), the code jumps to exception handler code while keeping interrupts disabled. -
The exception handler code is located here out of line to reduce performance impact on interrupts. The main body of the trampoline only handles interrupts.
-
If this was an interrupt, the trampoline entry code continues to save all the caller-save registers to the stack. This is done with interrupts disabled, as even if an interrupt arrived with a higher interrupt level it would still require all registers to be saved.
-
When
xnxtiis read here, the interrupt inputs to the CLIC might have changed from the time the handler was initially entered. The return value ofxnxti, which holds a pointer to an entry in the trap vector table, is saved in registera0so it can be passed as the first argument to the software-vectored interrupt handler, where it can be used to reconstruct the original interrupt id in the case where multiple vector entries use a common handler. There are multiple cases to consider, all of which are handled correctly by the definition ofxnxti:-
The II is still the ranking interrupt (no change). In this case, as the level of the II will still be higher than
pilfrom the OIC,xilandexccodewill be rewritten with the same value that they already had (effectively unchanged), andxnxtiwill return the table entry for the II. -
The II has been superceded by a higher-level non-SHV interrupt. In this case,
xilwill be set to the new higher interrupt level,exccodewill be updated to the new interrupt id, andxnxtiwill return the vector table entry for the new higher-level interrupt. The OIC is not disturbed, retaining the originalepcand the originalpil. This case reduces latency to service a more-important interrupt that arrives after the state-save sequence was begun for the less-important II. The II, if still pending-enabled, will be serviced sometime after the higher-level interrupt as described below. -
The II has been superceded by a higher-priority non-SHV interrupt at the same level. This operates similarly to the previous case, with
exccodeupdated to the new interrupt id. Because the lower-priority interrupt had not begun to run its service routine, this optimization preserves the property that interrupt handlers at the same interrupt level but different priorities execute atomically with respect to each other (i.e., they do not preempt each other). -
The II has disappeared and a lower-ranked non-SHV interrupt, which has interrupt level greater than the OIC’s
pilis present in CLIC. In this case, thexilof the handler will be reduced to the lower-ranked interrupt’s level,exccodewill be updated with the new interrupt id, andxnxtiwill return a pointer to the appropriate handler in table. In this case, the new lower-ranked interrupt would still have caused the original context to have been interrupted to run the handler, and the disappearing II has simply caused the lower-ranked interrupt’s entry and state-save sequence to begin earlier. -
The II has disappeared and either there is no current interrupt from the CLIC, or the current ranking interrupt is a non-SHV interrupt with level lower than {pil}. In this case, the
xilandexccodeare not updated, and 0 is returned byxnxti. The following trampoline code will then not fetch a vector from the table, and instead just restore the OIC context andmretback to it. This preserves the property that the OIC completes execution before servicing any new interrupt with a lower or equal interrupt level. -
The II has been superceded by a higher-level SHV interrupt. In this case, the
xilandexccodeare not updated, and 0 is returned byxnxti. Once interrupts are reenabled for the following instruction, the processor will preempt the current handler and execute the vectored interrupt at a higher interrupt level using the function pointer stored in the vector table.
-
-
Interrupts are now enabled. If a higher-level SHV interrupt had arrived while interrupts were disabled, then the current handler will be preempted and execution starts at the SHV handler address. If a non-vectored higher-level interrupt arrives now, it will also preempt the current handler and begin a nested state-save sequence at the handler entry point
irq_enter. -
The branch checks if the II disappeared or if a higher priority SHV at the same level appeared, in which case the current handler returns to the OIC. As most registers have not been touched, the routine can skip past most of the register restore code. This preserves the property that interrupts (SHV or non-SHV) at the same level do not preempt each other.
-
The value returned by
xnxtiis used to index the vector table and return the function pointer. -
This
csrrsiinstruction enables interrupts and is redundant when proceeding sequentially from the firstxnxtiread (6) or if looping back from the end of theservice_loop(13). However, it is required on the backward path from (16) to re-enable interrupts to allow preemption. It is scheduled after the table lookup to use what will often be a load-use delay slot. -
The
jalrinstruction actually calls the C ABI function that implements the handler. Interrupts are enabled at this point, so the C function can be preempted at any time by an interrupt with a higher level than currentxil. -
Once the handler returns, another read of
xnxtichecks if there are any more interrupts to service. Interrupts remain enabled. Thecsrrsiincludes a redundant set of thexieinterrupt enable to force the CSR instruction to update CSR state. Only non-SHV interrupts with a level greater thanpilwill be serviced in this loop. Note thatxilcan decrease from its current value on thexnxtiread.xilshould not increase in this code, as interrupts are enabled here and if a higher-level interrupt was ready, it should have preempted this instruction. -
If there was another appropriate interrupt to service, the code loops back to perform the next handler call. The
service_looponly contains 5 instructions, allowing multiple back-back interrupts to be handled without saving and restoring contexts. On a simple pipeline with a one-cycle load-use penalty, single-cycle CSR access, and a one-cycle taken-branch penalty, the service loop can initiate a new interrupt service with only 7 clock cycles of overhead per handler call. -
This instruction sequence restores the OIC. Interrupts are still enabled, so preemption is allowed during this restore.
-
Interrupts are disabled for the final steps of restoring the OIC, which requires loading
mcauseandmepcfrom the stacked values, and recovering the final register values from the OIC. -
A final read of
xnxtiis performed before returning, to reduce the maximum interrupt latency. If a suitable interrupt arrives, it can be serviced without saving context. Thecsrrciinstruction includes a redundant clear of the interrupt enable bit to ensure the CSR state updates occur. Interrupts must stay disabled until after the following branch to maintain the critical section used to restore the OIC in the case that there is no interrupt to service.
The following table summarizes the machine state changes that occur at
the first xnxti:
IC at entry |-> | at first nxti (6)
il CLIC | CLIC
level id V |-> mil code | level id V |-> mil code rd
p e<=p ? ? |-> | # Shouldn't happen
p e>p i 0 |-> e i | f>p j 0 |-> f j T # Same or superceded interrupt
p e>p i 0 |-> e i | f>p j 1 |-> e i 0 # Ignore vectored interrupt
p e>p i 0 |-> e i | f<=p j ? |-> e i 0 # Interrupt disappeared
p e>p i 1 |-> e i | # Won't be in trampolineThe overhead to save and restore registers in the interrupt trampoline can be reduced with a new embedded ABI that reduces the number of caller-save registers. Work is underway to define such an ABI, but it is likely to require around 7 integer registers to be saved/restored instead of 16 in the standard Unix ABI.
This will result in 18 instructions executed in the trampoline code
before arriving at the correct handler function, of which 9 are stores
(saving 7 registers plus 2 words for xepc and xcause).
The following analysis assumes a system with M-mode only and a new embedded ABI requiring 7 caller-save registers to be saved and restored. For cycle timings, we assume a simple 3-stage pipeline that has a one-cycle taken-branch or pipeline flush penalty, a one-cycle load-use delay, and single-cycle CSR access. This simple model ignores effects from contention in shared memory structures, or pipeline hazards from continuing long-latency operations in the interrupted code.
There are several cases to consider for the worst-case latency for a C-ABI higher-level interrupt handler that preempts lower-level code.
If an interrupt arrives while interrupts are enabled, either inside or
outside of a current handler, the processor will jump directly to
irq_enter at the new interrupt level. The system must flush the
execution pipeline and then execute 18 instructions, the last of which
is the jalr that calls the handler function. These 18 instructions
execute in 20 cycles using the simple pipeline model.
When interrupts are disabled, the arriving preempting handler could be
delayed. If the preempting interrupt arrives while interrupts are
disabled during the initial entry sequence (1)--(6), there will be no
additional delay as the first xnxti instruction (6) will cause the
higher-level interrupt handler to be invoked, replacing the original
interrupt cause.
If the preempting interrupt arrives after interrupts are disabled (15)
but before xnxti is read (16), then the trampoline will observe the
new interrupt during execution of the xnxti read (16), and take a
short branch back to the service_loop, which is lower latency than
the interrupt-disabled case.
If the preempting interrupt arrives after the read of xnxti commits
(16), then the interrupt has to wait an additional 4 instructions
until the mret reenables interrupts, at which point the interrupt
will be taken and the handler entered at irq_enter. In the simple
pipeline model, mret adds an additional pipeline flush cycle, so the
preemption latency is 20+5 cycles, which represents the worst-case for
a preempting C-ABI interrupt handler.
For many embedded systems, after initialization, essentially all code is run in response to an interrupt, interrupt levels are used to prioritize execution of different tasks, and the processor should sleep inbetween interrupt events to save energy.
The following code can be used as the background code that runs at
interrupt level 0 and which when there is no active work to do, puts
the processor to sleep with no active context, waiting for an
interrupt using the wfi instruction. The code is entered at the
enter_loop location and never returns directly.
# Source code for interrupt-driven model background code.
sleep:
csrrci x0, mstatus, MIE # Disable interrupts. (1)
wfi # Processor waits for next interrupt event.
csrrsi a0, mnxti, MIE # Gather interrupt details, and enable interrupts. (2)
beqz a0, sleep # Go back to sleep if no interrupt (will be preempted if SHV). (3)
service_loop: (4)
lw a1, (a0) # Get handler address.
csrrsi x0, mstatus, MIE # Enable interrupts
jalr a1 # Call C-ABI handler routine
csrrsi a0, mnxti, MIE # Claim any pending interrupt at level > 0
bnez a0, service_loop # Loop to service any interrupt.
# This is also entry point to begin sleeping.
enter_sleep: (5)
la a0, sleep
csrci x0, mstatus, MIE # Disable interrupts.
#--- Interrupts disabled
csrw mepc, a0 # Initialize mepc to point to sleep
li a0, (MMODE)<<PP|(0)<<PIL|(1)<<PIE
csrw mcause, a0 # Initialize mcause to have pp=M, pil=0, pie=1
mret # Jump to sleep at level 0 with interrupts enabled.
#--- Interrupts enabled-
The
sleeploop is used to stall the processor while waiting for work and is always entered at interrupt level 0. Interrupts are disabled, then awfiis executed. Thewfiwill stall the processor until some event occurs. When an event, including an interrupt occurs, thewfiretires. Because interrupts are disabled, the hart does not jump to an interrupt handler but instead executes the next instruction, avoiding context save/restore overhead. -
The read of
xnxtiwill determine if any non-SHV interrupt is available, and if so return a pointer to the table entry. Interrupts are enabled by this instruction to allow SHV interrupts to be taken via preemption. -
The value in
a0checked by the branch can be zero for two reasons. Either there was no interrupt detected or an SHV interrupt was detected. If there was no interrupt, the branch loops back to put the hart to sleep. Interrupts are enabled, so any SHV interrupt (which all have higher interrupt level than the current interrupt level of 0) will preempt the branch’s execution and call the SHV handler. Once the SHV handler returns, the branch will resume and cause execution to return back to thesleep_loop. -
The service loop is identical to that in the C-ABI interrupt handler, except that the previous interrupt level is 0, so all pending interrupts will be serviced in the loop before the loop exits. Interrupts are enabled, so preemption is allowed for both C-ABI trampoline and SHV interrupts. When an SHV interrupt at the same or lower interrupt level is the next to be serviced, the
xnxtiinstruction will return 0 causing execution to drop out of the loop. The following code will reinitialize the hart’s interrupt level to 0, and disable interrupts for one instruction, to ensure the SHV interrupt will be taken. -
This code initializes
mepcandmcausethen uses anmretto jump to thesleeploop while simultaneously reseting interrupt level to 0 and enabling interrupts. This is also the entry point to initiate interrupt-driven execution. Interrupts are enabled to allow SHV interrupts to preempt execution on the first instruction insleep(which disables interrupts again).
This code does not increase worst-case interrupt latency over that of the C-ABI trampoline.
Platforms may only implement non-vectored CLIC mode
without selective hardware vectoring
(cliccfg.nvbits=0), in which case, hardware vectoring can be emulated
by a single software trampoline present at NBASE using the separate
vector table address in xtvt. There are several different software
approaches possible, depending on system requirements and constraints,
as detailed in following subsections.
Where interrupts are known to be generated and handled in a single
privilege mode (i.e., M-mode only systems, or U-mode interrupt
handlers), a three-instruction sequence using the gp register to
hold the handler address can be used to indirect to an inline
interrupt handler of the type described in Inlines.
# Software-vectored interrupt servicing.
# Only safe for horizontal interrupts.
# Must be placed three instructions back from gp.
irq_enter:
csrrci gp, mnxti, MIE # Overwrite gp, keep interrupts disabled.
beqz gp, handle_exc # Encountered exception.
jalr gp, gp # Recreate gp and jump to handler.
gp: # Must be right before system's gp location.
# ... gp data section
# Must be within range of beqz instruction.
handle_exc:
# Has to recreate gp.The three-instruction sequence relies on the jalr instruction
recreating the value in the gp register, which is a known constant
pointing into the middle of the global data area, by placing the
jalr directly before the gp location in memory. The routine
jumped to by the jalr does not return via a j ra but instead ends
with an mret.
|
Note
|
This constraint on memory layout might not always be possible, particularly if the system does not allow placing executable memory right next to read-write memory, for example if the system does not allow a protection boundary to be placed at 'gp' and if executable code must not be writeable. |
The code can be used with preemptible inline interrupt handlers.
This section describes a more general software-trampoline scheme for
calling preemptible inline handlers, which factors out the
xepc/xcause save code into the trampoline, and which uses a
different interrupt handler calling convention.
The interrupt handlers for this scheme have a calling convention where
there is one caller-save argument register a0 that passes in the
handler address to distinguish different interrupt inputs, and one
temporary register a1 that is also caller-save. These two registers
had to be saved already by the trampoline. All other registers are
callee-save, except for the return address ra. The handler normally
returns with a regular j ra.
# Example handler with new calling convention.
# Only safe for horizontal interrupts.
# Handlers have two temporary registers available, a0, a1.
handler_example:
sw x0, INTERRUPT_FLAG, a0 # Clear interrupt flag.
la a0, COUNTER # Get counter address.
li a1, 1 # Increment value.
amoadd.w x0, (a0), a1 # Bump counter.
j ra
# Interrupt trampoline code.
irq_enter:
#----- Interrupts disabled on entry ---#
addi sp, sp, -FRAMESIZE # Create a frame on stack.
sw a0, OFFSET(sp) # Save working register.
csrr a0, mcause # Read cause.
bgez a0, handle_exc # Handler exception.
sw a1, OFFSET(sp) # Save working register.
csrr a1, mepc # Read epc.
sw a0, OFFSET(sp) # Save cause
csrrsi a0, mnxti, MIE # Get highest interrupt, enable interrupts.
#----- Interrupts enabled ---------#
beqz a0, exit
sw a1, OFFSET(sp) # Save epc.
sw ra, OFFSET(sp) # Save return address.
irq_loop:
lw a1, (a0) # Get function pointer.
jalr a1 # Call handler code.
csrrsi a0, mnxti, MIE # Get any next interrupt.
bnez a0, irq_loop # Service interrupt if any.
lw ra, OFFSET(sp) # Restore ra.
lw a1, OFFSET(sp) # Get epc.
exit:
lw a0, OFFSET(sp) # Get cause.
csrrci x0, mstatus, MIE # Disable interrupts.
#----- Interrupts disabled ---------#
csrw mepc, a1 # Put epc back.
lw a1, OFFSET(sp) # Restore a1.
csrw mcause, a0 # Put cause back.
lw a0, OFFSET(sp) # Restore a0.
addi sp, sp, FRAMESIZE # Free stack frame.
mret # Return from handler.
#------------------------------------#
handle_exc:
# ...
# Handle exception with interrupts disabled.
# ...
addi sp, sp, FRAMESIZE # Deallocate stack space
mret # Return from handler.
#------------------------------------#This interrupt handler can be used together with the wfi sleep
background routine shown above.
Interrupt handlers need to have a place to save the previous context’s
state to provide working registers for the handler code. If a handler
can be entered from a lower-privilege mode, a pointer to some safe
memory for the context save must be swapped in at entry to the
higher-privileged handler to avoid security holes. The RISC-V
privileged architecture provides the xscratch register to hold this
information for a higher-privilege mode while executing in a
lower-privilege mode. For the following discussion and code examples,
the assumption is that xscratch is used to hold the
higher-privilege-mode stack pointer but other software conventions are
possible (e.g., xscratch points to a thread context block).
Existing RISC-V ABIs allow addresses immediately below the stack
pointer to be overwritten by interrupt service routines. The current
stack pointer in sp (x2) should be swapped with xscratch whenever
a handler is entered from a lower-privilege mode, but should not be
swapped if entered from another handler in the same privilege mode,
including when preempting an existing interrupt handler. At exit from
a handler, the lower-privilege stack pointer should be swapped back in
if transitioning back to the lower-privilege mode.
In this convention, when code is running in a lower privilege mode,
xscratch holds the stack pointer for the higher-privilege mode. When
the higher-privilege mode is entered, xscratch is set to zero to
signal to any preempting handlers that the stack pointer has already
been swapped.
The old stack pointer is saved to new stack frame before new frame is created by bumping stack pointer, but this is done with interrupts disabled.
# This code is out of line to reduce worst-case preemption latency.
enter_M:
sw sp, OFFSET-FRAMESIZE(sp) # Save previous mscratch (M-mode sp)
addi sp, sp, -FRAMESIZE # Create a frame on stack.
sw a0, OFFSET(sp) # Save a register.
csrrw a0, mscratch, 0 # Get previous sp, and zero mscratch.
sw a0, OFFSET(sp) # Save previous sp (U-mode sp)
j continue # Jump back into handler
irq_enter:
#----- Interrupts disabled on entry ---#
csrrw sp, mscratch, sp # Swap stack pointer and scratch.
bnez mscratch, enter_M # Check if entering M-mode
csrrw sp, mscratch, sp # Already in M-mode, so swap sp back.
sw sp, OFFSET-FRAMESIZE(sp) # Save previous sp to stack.
addi sp, sp, -FRAMESIZE # Create a frame on stack.
sw x0, OFFSET(sp) # Save previous mscratch to stack (was zero).
sw a0, OFFSET(sp) # Save a register.
continue:
csrr a0, mcause # Read cause.
bgez a0, handle_exc # Handle exception.
sw a1, OFFSET(sp) # Save working register.
csrr a1, mepc # Read epc.
sw a0, OFFSET(sp) # Save cause
csrrsi a0, mnxti, MIE # Get highest interrupt, enable interrupts.
#----- Interrupts enabled ---------#
beqz a0, exit
...
#---- Critical section with interrupts disabled -----------------------
...
lw a0, OFFSET(sp) # Get previous mscratch.
csrw mscratch, a0 # Put back in mscratch.
lw a0, OFFSET(sp) # Restore original a0 value.
lw sp, OFFSET(sp) # Restore previous sp
mret # Return from interrupt.
#-----------------------------------------------------------------------This code can be used in a secure model where user-level code has one
stack, and all interrupts and exceptions are handled on a second
M-mode-only stack. In addition, background non-handler code in M-mode
can either use the same M-mode stack as the interrupt handler, or a
separate M-mode stack. The only difference is in the value held in
xscratch while the M-mode background thread is running (either 0 to
indicate use the existing stack pointer in sp or non-zero to
indicate this stack pointer should be used in the handler.
The above software scheme adds 7 instructions to the interrupt code path when preempting the same privilege mode, and adds an additional 6 instructions (13 total including two taken branches) for interrupts from a lower-privilege mode into a higher-privileged mode.
To accelerate interrupt handling with multiple privilege modes, a new
CSR xscratchcsw can be defined for all but the lowest privilege mode
to support conditional swapping of the xscratch register when
transitioning between privilege modes. The CSR instruction is used
once at the entry to a handler routine and once at handler exit, so
only adds two instructions to the interrupt code path. Though
designed to be used with csrrw instructions, these CSRs can be
accessed with any CSR instruction.
For all CSR instructions accessing xscratchcsw, the value written into
rd is either xscratch if xpp is different than the current
privilege mode, or rs1 if xpp is the same as the current privilege
mode. The xscratch register is only written if there is a privilege
mode difference, and if so, it is written obeying the usual CSR
read-modify-write conventions (e.g., swap/set/clear bits) using the
original xscratch value as one source operand and the other source
operand specified as usual in the instruction.
|
Note
|
This is different than a regular CSR instruction as the value returned is different from the value used in the read-modify-write operation. |
|
Note
|
The CSR instructions are defined to always copy a result
(xscratch or rs1) to the rd destination to simplify
implementations using register renaming, and in normal use the
instructions set both rs1 = sp and rd = sp.
|
csrrw rd, mscratchcsw, rs1
// Pseudocode operation.
if (mcause.mpp!=M-mode) then {
t = rs1; rd = mscratch; mscratch = t;
} else {
rd = rs1; // mscratch unchanged.
}
// Usual use: csrrw sp, mscratchcsw, sp|
Note
|
To avoid virtualization holes, software cannot directly read the
hart’s current privilege mode. The swap instruction will trap if
software tries to access a given mode’s xscratchcsw CSR from a
lesser-privileged mode, so the new CSR does not open a virtualization
hole.
|
Interrupt handlers running in the lowest privilege mode do not need to swap stack pointers, as they will only be entered by a horizontal interrupt from the same privilege mode. In systems with multiple privilege modes, handlers running in higher privilege modes must account for vertical interrupts taken from a lower privilege mode (in which case the stack pointer must be swapped) as well as horizontal interrupts from the same privilege mode.
# Example of inline interrupt with stack swapping.
.align 3
foo:
csrrw sp, mscratchcsw, sp # Conditionally swap in stack pointer.
addi sp, sp, -FRAMESIZE # Create a frame on stack.
sw s0, OFFSET(sp) # Save working register.
sw x0, INTERRUPT_FLAG, s0 # Clear interrupt flag.
sw s1, OFFSET(sp) # Save working register.
la s0, COUNTER # Get counter address.
li s1, 1
amoadd.w x0, (s0), s1 # Increment counter in memory.
lw s1, OFFSET(sp) # Restore registers.
lw s0, OFFSET(sp)
addi sp, sp, FRAMESIZE # Free stack frame.
csrrw sp, mscratchcsw, sp # Conditionally swap out stack pointer.
mret # Return from handler using saved mepc. # Example of inline preemptible interrupt with stack swapping.
.align 3
foo:
#----- Interrupts disabled on entry ---#
csrrw sp, mscratchcsw, sp # Conditionally swap in stack pointer.
addi sp, sp, -FRAMESIZE # Create a frame on stack.
sw s0, OFFSET(sp) # Save working register.
sw s1, OFFSET(sp) # Save working register.
csrr s0, mcause # Read cause.
csrr s1, mepc # Read epc.
csrrsi x0, mstatus, MIE # Enable interrupts.
#----- Interrupts enabled ---------#
sw s0, OFFSET(sp) # Save cause on stack.
sw x0, INTERRUPT_FLAG, s0 # Clear interrupt flag.
sw s1, OFFSET(sp) # Save epc on stack.
la s0, COUNTER # Get counter address.
li s1, 1
amoadd.w x0, (s0), s1 # Increment counter in memory.
lw s1, OFFSET(sp) # Restore epc
lw s0, OFFSET(sp) # Restore cause
#----- Interrupts disabled ---------#
csrrci x0, mstatus, MIE # Disable interrupts.
csrw mepc, s1 # Put epc back.
csrw mcause, s0 # Put cause back.
lw s1, OFFSET(sp) # Restore s1.
lw s0, OFFSET(sp) # Restore s0.
addi sp, sp, FRAMESIZE # Free stack frame.
csrrw sp, mscratchcsw, sp # Conditionally swap out stack pointer.
mret # Return from handler.
#------------------------------------# # Example C-ABI interrupt trampoline with stack swapping.
irq_enter:
#----
csrrw sp, mscratchcsw, sp # Conditionally swap in stack pointer.
addi sp, sp, -FRAMESIZE # Allocate space on stack.
# ...
# Everything else same as above.
# ...
addi sp, sp, FRAMESIZE # Reclaim stack space.
csrrw sp, mscratchcsw, sp # Conditionally swap back stack pointer.
mret # Return from interrupt.
#-----------------------------------------------------------------------
#-----------------------------------------------------------------------
handle_exc:
# ...
# Perform exception processing with interrupts disabled
# ...
addi sp, sp, FRAMESIZE # Reclaim stack space.
csrrw sp, mscratchcsw, sp # Conditionally swap back stack pointer.
mret # Return from exception
#----------------------------------------------------------------------In all cases, conditionally swapping the stack to account for potential privilege-mode changes adds two extra instructions to all interrupt handlers.
Within a single privilege mode, it can be useful to separate interrupt handler tasks from application tasks to increase robustness, reduce space usage, and aid in system debugging. Interrupt handler tasks have non-zero interrupt levels, while application tasks have an interrupt level of zero.
A new xscratchcswl CSR is added to support faster swapping of the
stack pointer between interrupt and non-interrupt code running in the
same privilege mode.
csrrw rd, mscratchcswl, rs1
// Pseudocode operation.
if ( (mcause.pil==0) != (minstatus.mil==0) ) then {
t = rs1; rd = mscratch; mscratch = t;
} else {
rd = rs1; // mscratch unchanged.
}
// Usual use: csrrw sp, mscratchcswl, spThis new CSR operates similarly to xscratchcsw except that the swap
condition is true when the interrupter and interruptee are not both
application tasks or not both interrupt handlers.
The original basic mode interrupts retain their interrupt ID in CLIC mode.
The clicintctl settings are now used to delegate these interrupts as
required.
An additional CLIC software interrupt bit (csip) is provided. This is generally available for software use, but is usually used for the local background interrupt thread.
CLIC interrupt inputs are allocated IDs beginning at interrupt ID 16. Any fast local interrupts that would have been connected at interrupt ID 16 and above should now be mapped into corresponding inputs of the CLIC.
ID Interrupt Note
0 usip User software Interrupt
1 ssip Supervisor software Interrupt
2 reserved
3 msip Machine software interrupt
4 utip User timer interrupt
5 stip Supervisor timer interrupt
6 reserved
7 mtip Machine timer interrupt
8 ueip User external (PLIC) interrupt
9 seip Supervisor external (PLIC) interrupt
10 reserved
11 meip Machine external (PLIC) interrupt
12 csip CLIC software interrupt
13 reserved
14 reserved
15 reserved
16+ inputs CLIC external inputs