On Mon, Jun 15, 2020 at 4:07 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> Start a new TSC synchronization generation whenever the
> IA32_TIME_STAMP_COUNTER MSR is written on a vCPU that has already
> participated in the current TSC synchronization generation.
>
> Previously, it was not possible to restore the IA32_TIME_STAMP_COUNTER
> MSR to a value less than the TSC frequency. Since vCPU initialization
> sets the IA32_TIME_STAMP_COUNTER MSR to zero, a subsequent
> KVM_SET_MSRS ioctl that attempted to write a small value to the
> IA32_TIME_STAMP_COUNTER MSR was viewed as an attempt at TSC
> synchronization. Notably, this was the case even for single vCPU VMs,
> which were always synchronized.
>
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> Reviewed-by: Peter Shier <pshier@xxxxxxxxxx>
> Reviewed-by: Oliver Upton <oupton@xxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 13 +++++--------
> 1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9e41b5135340..2555ea2cd91e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2015,7 +2015,6 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
> u64 offset, ns, elapsed;
> unsigned long flags;
> bool matched;
> - bool already_matched;
> u64 data = msr->data;
> bool synchronizing = false;
>
> @@ -2032,7 +2031,8 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
> * kvm_clock stable after CPU hotplug
> */
> synchronizing = true;
> - } else {
> + } else if (vcpu->arch.this_tsc_generation !=
> + kvm->arch.cur_tsc_generation) {
> u64 tsc_exp = kvm->arch.last_tsc_write +
> nsec_to_cycles(vcpu, elapsed);
> u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
> @@ -2062,7 +2062,6 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
> offset = kvm_compute_tsc_offset(vcpu, data);
> }
> matched = true;
> - already_matched = (vcpu->arch.this_tsc_generation == kvm->arch.cur_tsc_generation);
> } else {
> /*
> * We split periods of matched TSC writes into generations.
> @@ -2102,12 +2101,10 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
> raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
>
> spin_lock(&kvm->arch.pvclock_gtod_sync_lock);
> - if (!matched) {
> - kvm->arch.nr_vcpus_matched_tsc = 0;
> - } else if (!already_matched) {
> + if (matched)
> kvm->arch.nr_vcpus_matched_tsc++;
> - }
> -
> + else
> + kvm->arch.nr_vcpus_matched_tsc = 0;
> kvm_track_tsc_matching(vcpu);
> spin_unlock(&kvm->arch.pvclock_gtod_sync_lock);
> }
> --
> 2.27.0.290.gba653c62da-goog
>
Ping.
