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x86: pvclock: add note about rdtsc barriers

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As noted by Gleb, not advertising SSE2 support implies
no RDTSC barriers.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Marcelo Tosatti 2012-11-27 23:28:53 -02:00
parent 2697902be8
commit 189e11731a
1 changed files with 6 additions and 0 deletions
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6
arch/x86/include/asm/pvclock.h
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@ -74,6 +74,12 @@ unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
u8 ret_flags;
version = src->version;
/* Note: emulated platforms which do not advertise SSE2 support
* result in kvmclock not using the necessary RDTSC barriers.
* Without barriers, it is possible that RDTSC instruction reads from
* the time stamp counter outside rdtsc_barrier protected section
* below, resulting in violation of monotonicity.
*/
rdtsc_barrier();
offset = pvclock_get_nsec_offset(src);
ret = src->system_time + offset;