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Commit 189e1173 authored by Marcelo Tosatti's avatar Marcelo Tosatti
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x86: pvclock: add note about rdtsc barriers 



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

Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 2697902b

arch/x86/include/asm/pvclock.h

+6 −0
Original line number Diff line number Diff line
@@ -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;