x86: kvmclock: drop rdtsc_barrier()

Drop unnecessary rdtsc_barrier(), as has been determined empirically,
see 057e6a8c660e95c3f4e7162e00e2fee1fc90c50d for details.

Noticed by Andy Lutomirski.

Improves clock_gettime() by approximately 15% on
Intel i7-3520M @ 2.90GHz.

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

diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index d6b078e9fa28a3f4588237cb9a122f5b5ce53162..628954ceede1159fa7df62944d7b140df73d629b 100644 (file)
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -86,7 +86,6 @@ unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
        offset = pvclock_get_nsec_offset(src);
        ret = src->system_time + offset;
        ret_flags = src->flags;
-       rdtsc_barrier();
 
        *cycles = ret;
        *flags = ret_flags;