kvm PTP: drop TSC correction from ptp_kvm_gettime

The TSC correction in ptp_kvm_gettime was an attempt to handle high load situations. Unfortunately it is incorrect because userspace expects PTP clock reads to happen in the middle point between realtime clock reads:

	|* | | | | | | | | | | | |* | | | | | | | | | | | |* |
	rt-read-1	ptp-clock-read-1	rt-read-2

The TSC correction causes the PTP clock reads to read the clock at:

	|* | | | | | | | | | | | | | | | | |* | | | | | | |* |
	rt-read-1	ptp-clock-read-1	rt-read-2

So drop the TSC offset correction, increasing precision of synchronization of an idle guest to <= 10ns.

Signed-off-by Marcelo Tosatti <[email protected]>

drivers/ptp/ptp_kvm.c +10 -50

index 3ea41b8..994aa04 100644
@@ -123,61 +123,21 @@ static int ptp_kvm_gettime(struct ptp_clock_info *ptp, struct timespec64 *ts)
 {
 	unsigned long ret;
 	struct timespec64 tspec;
-	u64 delta, offset;
-	unsigned version;
-	int cpu;
-	struct pvclock_vcpu_time_info *src;
 
 	spin_lock(&kvm_ptp_lock);
 
-	preempt_disable_notrace();
-	cpu = smp_processor_id();
-	src = &hv_clock[cpu].pvti;
-
-	do {
-		/*
-		 * We are measuring the delay between
-		 * kvm_hypercall and rdtsc using TSC,
-		 * and converting that delta through
-		 * tsc_to_system_mul and tsc_shift
-		 * So any changes to tsc_to_system_mul
-		 * and tsc_shift in this region
-		 * invalidate the measurement.
-		 */
-		version = pvclock_read_begin(src);
-
-		ret = kvm_hypercall2(KVM_HC_CLOCK_PAIRING,
-				     clock_pair_gpa,
-				     KVM_CLOCK_PAIRING_WALLCLOCK);
-		if (ret != 0) {
-			pr_err_ratelimited("clock pairing hypercall ret %lu\n", ret);
-			spin_unlock(&kvm_ptp_lock);
-			preempt_enable_notrace();
-			return -EOPNOTSUPP;
-		}
-
-		tspec.tv_sec = clock_pair.sec;
-		tspec.tv_nsec = clock_pair.nsec;
-
-		delta = rdtsc_ordered() - clock_pair.tsc;
-		offset = pvclock_scale_delta(delta, src->tsc_to_system_mul,
-					     src->tsc_shift);
-	} while (pvclock_read_retry(src, version));
-
-	preempt_enable_notrace();
-
-	spin_unlock(&kvm_ptp_lock);
-
-	while (unlikely(offset >= NSEC_PER_SEC)) {
-		tspec.tv_sec++;
-		offset -= NSEC_PER_SEC;
+	ret = kvm_hypercall2(KVM_HC_CLOCK_PAIRING,
+			     clock_pair_gpa,
+			     KVM_CLOCK_PAIRING_WALLCLOCK);
+	if (ret != 0) {
+		pr_err("clock offset hypercall ret %lu\n", ret);
+		spin_unlock(&kvm_ptp_lock);
+		return -EOPNOTSUPP;
 	}
 
-	tspec.tv_nsec = tspec.tv_nsec + offset;
-	if (tspec.tv_nsec >= NSEC_PER_SEC) {
-		tspec.tv_sec++;
-		tspec.tv_nsec -= NSEC_PER_SEC;
-	}
+	tspec.tv_sec = clock_pair.sec;
+	tspec.tv_nsec = clock_pair.nsec;
+	spin_unlock(&kvm_ptp_lock);
 
 	memcpy(ts, &tspec, sizeof(struct timespec64));
 

The TSC correction in ptp_kvm_gettime was an attempt to handle high load situations. Unfortunately it is incorrect because userspace expects PTP clock reads to happen in the middle point between realtime clock reads:

	|* | | | | | | | | | | | |* | | | | | | | | | | | |* |
	rt-read-1	ptp-clock-read-1	rt-read-2

The TSC correction causes the PTP clock reads to read the clock at:

	|* | | | | | | | | | | | | | | | | |* | | | | | | |* |
	rt-read-1	ptp-clock-read-1	rt-read-2

So drop the TSC offset correction, increasing precision of synchronization of an idle guest to <= 10ns.

Signed-off-by Marcelo Tosatti <[email protected]>