Re: [PATCH 09/18] Robust TSC compensation

Marcelo Tosatti
2010-07-13T20:34:18Z

On Mon, Jul 12, 2010 at 04:25:29PM -1000, Zachary Amsden wrote:
> Make the match of TSC find TSC writes that are close to each other
> instead of perfectly identical; this allows the compensator to also
> work in migration / suspend scenarios.
>
> Signed-off-by: Zachary Amsden <zamsden@redhat.com>
> ---
> arch/x86/kvm/x86.c | 14 ++++++++++----
> 1 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 79c4608..51d3f3e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -926,21 +926,27 @@ void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
> struct kvm *kvm = vcpu->kvm;
> u64 offset, ns, elapsed;
> struct timespec ts;
> + s64 sdiff;
>
> spin_lock(&kvm->arch.tsc_write_lock);
> offset = data - native_read_tsc();
> ns = get_kernel_ns();
> elapsed = ns - kvm->arch.last_tsc_nsec;
> + sdiff = data - kvm->arch.last_tsc_write;
> + if (sdiff < 0)
> + sdiff = -sdiff;
>
> /*
> - * Special case: identical write to TSC within 5 seconds of
> + * Special case: close write to TSC within 5 seconds of
> * another CPU is interpreted as an attempt to synchronize
> - * (the 5 seconds is to accomodate host load / swapping).
> + * The 5 seconds is to accomodate host load / swapping as
> + * well as any reset of TSC during the boot process.
> *
> * In that case, for a reliable TSC, we can match TSC offsets,
> - * or make a best guest using kernel_ns value.
> + * or make a best guest using elapsed value.
> */
> - if (data == kvm->arch.last_tsc_write && elapsed < 5ULL * NSEC_PER_SEC) {
> + if (sdiff < nsec_to_cycles(5ULL * NSEC_PER_SEC) &&
> + elapsed < 5ULL * NSEC_PER_SEC) {
> if (!check_tsc_unstable()) {
> offset = kvm->arch.last_tsc_offset;
> pr_debug("kvm: matched tsc offset for %llu\n", data);

What prevents a vcpu from seeing its TSC go backwards, in case the first
write in the 5 second window is smaller than the victim vcpu's last
visible TSC value ?