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authorMarcelo Tosatti <mtosatti@redhat.com>2012-11-27 23:28:53 -0200
committerMarcelo Tosatti <mtosatti@redhat.com>2012-11-27 23:29:08 -0200
commit189e11731aa858597095fbe1e6d243bad26bd96b (patch)
tree7d7857fc8766606afb45da7303fa3242ae844665 /arch/x86/include
parent2697902be89d7f38e9736dfe946cd0e5d4f47b80 (diff)
downloadop-kernel-dev-189e11731aa858597095fbe1e6d243bad26bd96b.zip
op-kernel-dev-189e11731aa858597095fbe1e6d243bad26bd96b.tar.gz
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>
Diffstat (limited to 'arch/x86/include')
-rw-r--r--arch/x86/include/asm/pvclock.h6
1 files changed, 6 insertions, 0 deletions
diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index ea27a8d..63f9167 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;
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