2017-02-08 11:31-0200, Marcelo Tosatti:
>
> 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 <mtosatti@xxxxxxxxxx>
Can't say no to the diffstat, :)
Reviewed-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
(The subject tag should be different, though, "ptp_kvm:"?)
> diff --git a/drivers/ptp/ptp_kvm.c b/drivers/ptp/ptp_kvm.c
> index 3ea41b8..994aa04 100644
> --- a/drivers/ptp/ptp_kvm.c
> +++ b/drivers/ptp/ptp_kvm.c
> @@ -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));
>
