Merge bitcoin#26839: Add support for RNDR/RNDRRS for AArch64 on Linux

aee5404 Add support for RNDR/RNDRRS for aarch64 on Linux (John Moffett)

Pull request description:

  This checks whether the ARMv8.5-A optional TRNG extensions [RNDR](https://developer.arm.com/documentation/ddi0601/2022-12/AArch64-Registers/RNDR--Random-Number) and [RNDRRS](https://developer.arm.com/documentation/ddi0601/2022-12/AArch64-Registers/RNDRRS--Reseeded-Random-Number) are available and, if they are, uses them for random entropy purposes.

  They are nearly functionally identical to the x86 RDRAND/RDSEED extensions and are used in a similar manner.

  Currently, there [appears to be](https://marcin.juszkiewicz.com.pl/download/tables/arm-socs.html) only one actual hardware implementation -- the Amazon Graviton 3. (See the `rnd` column in the link.) However, future hardware implementations may become available.

  It's not possible to directly query for the capability in userspace, but the Linux kernel [added support](torvalds/linux@1a50ec0) for querying the extension via `getauxval` in version 5.6 (in 2020), so this is limited to Linux-only for now.

  Reviewers may want to launch any of the `c7g` instances from AWS to test the Graviton 3 hardware. Alternatively, QEMU emulates these opcodes for `aarch64` with CPU setting `max`.

  Output from Graviton 3 hardware:

  ```
  ubuntu@ip:~/bitcoin$ src/bitcoind -regtest
  2023-01-06T20:01:48Z Bitcoin Core version v24.99.0-3670266ce89a (release build)
  2023-01-06T20:01:48Z Using the 'arm_shani(1way,2way)' SHA256 implementation
  2023-01-06T20:01:48Z Using RNDR and RNDRRS as additional entropy sources
  2023-01-06T20:01:48Z Default data directory /home/ubuntu/.bitcoin
  ```

  Graviton 2 (doesn't support extensions):

  ```
  ubuntu@ip:~/bitcoin$ src/bitcoind -regtest
  2023-01-06T20:05:04Z Bitcoin Core version v24.99.0-3670266ce89a (release build)
  2023-01-06T20:05:04Z Using the 'arm_shani(1way,2way)' SHA256 implementation
  2023-01-06T20:05:04Z Default data directory /home/ubuntu/.bitcoin
  ```

  This partially closes bitcoin#26796. As noted in that issue, OpenSSL [added support](openssl/openssl#15361) for these extensions a little over a year ago.

ACKs for top commit:
  achow101:
    ACK aee5404
  laanwj:
    Tested ACK aee5404

Tree-SHA512: 1c1eb345d6690f5307a87e9bac8f06a0d1fdc7ca35db38fa22192510a44289a03252e4677dc7cbf731a27e6e3a9a4e42b6eb4149fe063bc1c905eb2536cdb1d3

src/random.cpp

@@ -38,6 +38,9 @@
 #ifdef HAVE_SYSCTL_ARND
 #include <sys/sysctl.h>
 #endif
+#if defined(HAVE_STRONG_GETAUXVAL) && defined(__aarch64__)
+#include <sys/auxv.h>
+#endif
 
 [[noreturn]] static void RandFailure()
 {
@@ -173,6 +176,62 @@ static uint64_t GetRdSeed() noexcept
 #endif
 }
 
+#elif defined(__aarch64__) && defined(HWCAP2_RNG)
+
+static bool g_rndr_supported = false;
+
+static void InitHardwareRand()
+{
+    if (getauxval(AT_HWCAP2) & HWCAP2_RNG) {
+        g_rndr_supported = true;
+    }
+}
+
+static void ReportHardwareRand()
+{
+    // This must be done in a separate function, as InitHardwareRand() may be indirectly called
+    // from global constructors, before logging is initialized.
+    if (g_rndr_supported) {
+        LogPrintf("Using RNDR and RNDRRS as additional entropy sources\n");
+    }
+}
+
+/** Read 64 bits of entropy using rndr.
+ *
+ * Must only be called when RNDR is supported.
+ */
+static uint64_t GetRNDR() noexcept
+{
+    uint8_t ok;
+    uint64_t r1;
+    do {
+        // https://developer.arm.com/documentation/ddi0601/2022-12/AArch64-Registers/RNDR--Random-Number
+        __asm__ volatile("mrs %0, s3_3_c2_c4_0; cset %w1, ne;"
+                         : "=r"(r1), "=r"(ok)::"cc");
+        if (ok) break;
+        __asm__ volatile("yield");
+    } while (true);
+    return r1;
+}
+
+/** Read 64 bits of entropy using rndrrs.
+ *
+ * Must only be called when RNDRRS is supported.
+ */
+static uint64_t GetRNDRRS() noexcept
+{
+    uint8_t ok;
+    uint64_t r1;
+    do {
+        // https://developer.arm.com/documentation/ddi0601/2022-12/AArch64-Registers/RNDRRS--Reseeded-Random-Number
+        __asm__ volatile("mrs %0, s3_3_c2_c4_1; cset %w1, ne;"
+                         : "=r"(r1), "=r"(ok)::"cc");
+        if (ok) break;
+        __asm__ volatile("yield");
+    } while (true);
+    return r1;
+}
+
 #else
 /* Access to other hardware random number generators could be added here later,
  * assuming it is sufficiently fast (in the order of a few hundred CPU cycles).
@@ -191,6 +250,12 @@ static void SeedHardwareFast(CSHA512& hasher) noexcept {
         hasher.Write((const unsigned char*)&out, sizeof(out));
         return;
     }
+#elif defined(__aarch64__) && defined(HWCAP2_RNG)
+    if (g_rndr_supported) {
+        uint64_t out = GetRNDR();
+        hasher.Write((const unsigned char*)&out, sizeof(out));
+        return;
+    }
 #endif
 }
 
@@ -216,6 +281,14 @@ static void SeedHardwareSlow(CSHA512& hasher) noexcept {
         }
         return;
     }
+#elif defined(__aarch64__) && defined(HWCAP2_RNG)
+    if (g_rndr_supported) {
+        for (int i = 0; i < 4; ++i) {
+            uint64_t out = GetRNDRRS();
+            hasher.Write((const unsigned char*)&out, sizeof(out));
+        }
+        return;
+    }
 #endif
 }