x86: pvclock: add note about rdtsc barriers

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

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index ea27a8dd189..63f91679af7 100644
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -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;