diff options
| -rw-r--r-- | arch/x86/kernel/tsc.c | 9 |
1 files changed, 5 insertions, 4 deletions
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index db697b81b8b1..1e6244202612 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -769,13 +769,14 @@ static cycle_t __vsyscall_fn vread_tsc(void) cycle_t ret; /* - * Surround the RDTSC by barriers, to make sure it's not - * speculated to outside the seqlock critical section and - * does not cause time warps: + * Empirically, a fence (of type that depends on the CPU) + * before rdtsc is enough to ensure that rdtsc is ordered + * with respect to loads. The various CPU manuals are unclear + * as to whether rdtsc can be reordered with later loads, + * but no one has ever seen it happen. */ rdtsc_barrier(); ret = (cycle_t)vget_cycles(); - rdtsc_barrier(); return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ? ret : VVAR(vsyscall_gtod_data).clock.cycle_last; |