pvclock: introduce seqcount-like API

The version field in struct pvclock_vcpu_time_info basically implements
a seqcount.  Wrap it with the usual read_begin and read_retry functions,
and use these APIs instead of peppering the code with smp_rmb()s.
While at it, change it to the more pedantically correct virt_rmb().

With this change, __pvclock_read_cycles can be simplified noticeably.

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

arch/x86/entry/vdso/vclock_gettime.c

@@ -123,9 +123,7 @@ static notrace cycle_t vread_pvclock(int *mode)
 	 */
 
 	do {
-		version = pvti->version;
-
-		smp_rmb();
+		version = pvclock_read_begin(pvti);
 
 		if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
 			*mode = VCLOCK_NONE;
@@ -137,10 +135,7 @@ static notrace cycle_t vread_pvclock(int *mode)
 		pvti_tsc_shift = pvti->tsc_shift;
 		pvti_system_time = pvti->system_time;
 		pvti_tsc = pvti->tsc_timestamp;
-
-		/* Make sure that the version double-check is last. */
-		smp_rmb();
-	} while (unlikely((version & 1) || version != pvti->version));
+	} while (pvclock_read_retry(pvti, version));
 
 	delta = tsc - pvti_tsc;
 	ret = pvti_system_time +