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CycleClock: use RDTIME instead of RDCYCLE on RISC-V (#1727) 

Starting with Linux 6.6 [1], RDCYCLE is a privileged instruction on
RISC-V and can't be used directly from userland. There is a sysctl
option to change that as a transition period, but it will eventually
disappear.

Use RDTIME instead, which while less accurate has the advantage of being
synchronized between CPU (and thus monotonic) and of constant frequency.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc4c07c89aada16229084eeb93895c95b7eabaa3

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>
This commit is contained in:
aurel32 2024-01-04 10:16:40 +01:00 committed by GitHub
parent e523e454f2
commit 99bdb2127d
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1 changed files with 6 additions and 5 deletions
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11
src/cycleclock.h
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@ -189,15 +189,16 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
#endif
return tsc;
#elif defined(__riscv) // RISC-V
// Use RDCYCLE (and RDCYCLEH on riscv32)
// Use RDTIME (and RDTIMEH on riscv32).
// RDCYCLE is a privileged instruction since Linux 6.6.
#if __riscv_xlen == 32
uint32_t cycles_lo, cycles_hi0, cycles_hi1;
// This asm also includes the PowerPC overflow handling strategy, as above.
// Implemented in assembly because Clang insisted on branching.
asm volatile(
"rdcycleh %0\n"
"rdcycle %1\n"
"rdcycleh %2\n"
"rdtimeh %0\n"
"rdtime %1\n"
"rdtimeh %2\n"
"sub %0, %0, %2\n"
"seqz %0, %0\n"
"sub %0, zero, %0\n"
@ -206,7 +207,7 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
return (static_cast<uint64_t>(cycles_hi1) << 32) | cycles_lo;
#else
uint64_t cycles;
asm volatile("rdcycle %0" : "=r"(cycles));
asm volatile("rdtime %0" : "=r"(cycles));
return cycles;
#endif
#elif defined(__e2k__) || defined(__elbrus__)