diff options
| -rw-r--r-- | arch/x86/kernel/tsc.c | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 1e62442..24249a5 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -767,6 +767,7 @@ static cycle_t read_tsc(struct clocksource *cs) static cycle_t __vsyscall_fn vread_tsc(void) { cycle_t ret; + u64 last; /* * Empirically, a fence (of type that depends on the CPU) @@ -778,8 +779,21 @@ static cycle_t __vsyscall_fn vread_tsc(void) rdtsc_barrier(); ret = (cycle_t)vget_cycles(); - return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ? - ret : VVAR(vsyscall_gtod_data).clock.cycle_last; + last = VVAR(vsyscall_gtod_data).clock.cycle_last; + + if (likely(ret >= last)) + return ret; + + /* + * GCC likes to generate cmov here, but this branch is extremely + * predictable (it's just a funciton of time and the likely is + * very likely) and there's a data dependence, so force GCC + * to generate a branch instead. I don't barrier() because + * we don't actually need a barrier, and if this function + * ever gets inlined it will generate worse code. + */ + asm volatile (""); + return last; } #endif |
