Pass through and record PR_SVE system calls on arm

[earthling-amzn]

Hotspot's JIT compilers use these system calls on ARMv8 to configure and detect SVE registers. These are not currently supported by rr. We could workaround this in the JVM by setting -XX:UseSVE=0, but it would also be nice to have rr support this.

[khuey]

Out of curiosity and my own lack of familiarity with SVE, is the "correct" value of PR_SVE_GET_VL easily observable by executing some SVE instruction(s)?

[earthling-amzn]

Here's a patch for the test case in this PR that, I think, shows what you are asking:

diff --git a/src/test/arm/sve_vl.c b/src/test/arm/sve_vl.c
index 447dabe4..9056c4a3 100644
--- a/src/test/arm/sve_vl.c
+++ b/src/test/arm/sve_vl.c
@@ -35,6 +35,12 @@ int main(void) {
   int vl2 = prctl(PR_SVE_GET_VL);
   test_assert((vl2 & PR_SVE_VL_LEN_MASK) == len);
 
+  /* Verify the prctl value matches the hardware vector length.
+     RDVL returns the SVE vector length in bytes for the current thread. */
+  long rdvl;
+  __asm__ volatile(".arch armv8-a+sve\n\trdvl %0, #1" : "=r"(rdvl));
+  test_assert(rdvl == len);
+
   atomic_puts("EXIT-SUCCESS");
   return 0;
 }

It adds an assertion that the value returned from prctl(PR_SVE_GET_VL) matches what the instruction to read vector length rdvl returns. On my host (graviton-3), the vector length is 32 bytes (256 bits).