x86/asm/tsc, x86/kvm: Drop open-coded barrier and use rdtsc_ordered() in kvmclock

__pvclock_read_cycles() used to have two barriers, one of which was unnecessary,
which got removed after an initial version of this patch was sent.

But the barrier is still open-coded unnecessarily - get rid of
that barrier and clean up the code by just using rdtsc_ordered().

Signed-off-by: Andy Lutomirski <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Huang Rui <[e-mail]>
Cc: John Stultz <[e-mail]>
Cc: Len Brown <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Marcelo Tosatti <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Radim Krcmar <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: kvm ML <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
[ Ported it to v4.2-rc1. ]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/pvclock.h

···
 static __always_inline
 u64 pvclock_get_nsec_offset(const struct pvclock_vcpu_time_info *src)
 {
-	u64 delta = rdtsc() - src->tsc_timestamp;
+	u64 delta = rdtsc_ordered() - src->tsc_timestamp;
 	return pvclock_scale_delta(delta, src->tsc_to_system_mul,
 				   src->tsc_shift);
 }
···
 	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;
 	ret_flags = src->flags;