p4c-dpdk back end does not support 128-bit arithmetic #4174
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Topics related to the DPDK back end
enhancement
This topic discusses an improvement to existing compiler code.
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jafingerhut
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dpdk
This was referenced Sep 23, 2023
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PR for supporting 128 bitwise operations on dpdk compiler backend. #4952 |
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Looks like 4952 in p4lang/p4c is Merged. Can this one be updated? |
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The failures are not completely resolved yet and the program is still marked XFail: #4952 (comment) describes the remaining work to do. |
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For anyone looking for the gory details, basically the PR #4952 did add a lot of support for 128-bit operations in p4lang/p4c DPDK back end. However, even with that PR it still does not support 128-bit constant values in the P4 source program, which the DASH program does use some of. The bad news is that there are still 2 error messages (shown below) with a 2024-Oct-31 version of p4c source code. The good news is that the number of error messages is down from something-over-20 down to 2 with the changes in PR #4952 |
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Nice work!!! (error message decrease) 😊
From: Andy Fingerhut ***@***.***>
Sent: Tuesday, November 5, 2024 4:11 PM
To: p4lang/p4c ***@***.***>
Cc: Kristina Moore ***@***.***>; Comment ***@***.***>
Subject: Re: [p4lang/p4c] p4c-dpdk back end does not support 128-bit arithmetic (Issue #4174)
For anyone looking for the gory details, basically the PR #4952<#4952> did add a lot of support for 128-bit operations in p4lang/p4c DPDK back end. However, even with that PR it still does not support 128-bit constant values in the P4 source program, which the DASH program does use some of. The bad news is the 2 error messages shown below with a 2024-Oct-31 version of p4c source code. The good news is that the number of error messages is down from something-over-20 down to 2 with the changes in PR #4952<#4952>
$ git clone https://github.com/sonic-net/DASH
$ cd DASH
$ git log -n 1 | head -n 3
commit ee1fa2350a444409e20ad825b13fafdf67010e41
Author: Shaofeng Wu ***@***.******@***.***>>
Date: Tue Nov 5 17:24:32 2024 +0800
$ cd dash-pipeline/bmv2
$ mkdir -p output
$ $HOME/forks/p4c/build/p4c-dpdk \
-DTARGET_DPDK_PNA -DPNA_CONNTRACK \
…--arch pna \
--p4runtime-files output/dash_pipeline.p4info.txtpb \
--bf-rt-schema output/dash_pipeline.bfrt.json \
-o output/dash_pipeline.spec \
--pp output/dash_pipeline.pp.p4 \
dash_pipeline.p4
[ ... many warning messages omitted ... ]
[--Werror=overlimit] error: DPDK target supports up-to 64-bit immediate values, 128w0xffffffffffffffffffffffff exceeds the limit
[--Werror=overlimit] error: DPDK target supports up-to 64-bit immediate values, 128w0xffffffffffffffffffffffff exceeds the limit
stages/../dash_routing_types.p4(173): [--Werror=overlimit] error: DPDK target supports up-to 64-bit immediate values, 128w0xffffffffffffffffffffffff exceeds the limit
inout metadata_t meta,
^^^^
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With commit #4952, p4c-dpdk supports many operations on 128-bit values.
The one known remaining omission is that it does not support 128-bit constant literal values, which are used by the current DASH program, and seem likely to be desirable to use in many P4 programs that manipulate 128-bit values.
The text of the original issue from 2023-Sep-23 is given below, for historical reference:
With this p4c commit #4171 the p4c-dpdk back end at least usually correctly reports when it does not support a P4 program that uses arithmetic operations on operands larger than 64 bits in size.
This issue is meant to track one possible enhancement that would enable a larger class of useful P4 programs to compile and run on P4-DPDK, which is: implement arithmetic on operands up to 128 bits in size in the P4-DPDK software switch, and then modify the p4c-dpdk back end compiler to take advantage of these operations.
Motivation: IPv6 addresses are commonly manipulated data values in P4 programs, and often arithmetic operations are performed on these values, especially these operations:
(my_ipv6_addr == some_128_bit_constant)my_ipv6_addr[31:0] = some_bit_32_expressionI believe that implementing at least the operations above would enable the DASH P4 code to compile without changes. For example, see the commands in issue #4156
I have attempted to modify the P4 code mentioned there to work with the current p4c-dpdk back end and P4-DPDK software switch as of 2023-Sep-23, but I do not think this is feasible. It is necessary to at least be able to do an
lpmmatch kind in a table key on a full 128-bit IPv6 address, and also to compare a 128-bit value to 0, and to do bit-wise AND, OR, and XOR. There is no practical way in P4 to do anlpmmatch on two 64-bit fields that works the same way as a 128-bitlpmmatch.I would recommend that the P4-DPDK back end be enhanced to support all of the following operations, too, on any values up to 128 bits wide:
!=,<,<=,>,>=at least on unsigned bit strings, but ideally also for signed bit strings.The text was updated successfully, but these errors were encountered: