Re: [PATCH 16/17] TSC reset compensation

Marcelo Tosatti
2010-06-16T00:27:32Z

On Mon, Jun 14, 2010 at 09:34:18PM -1000, Zachary Amsden wrote:
> Attempt to synchronize TSCs which are reset to the same value. In the
> case of a reliable hardware TSC, we can just re-use the same offset, but
> on non-reliable hardware, we can get closer by adjusting the offset to
> match the elapsed time.
>
> Signed-off-by: Zachary Amsden <zamsden@redhat.com>
> ---
> arch/x86/kvm/x86.c | 34 ++++++++++++++++++++++++++++++++--
> 1 files changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8e836e9..cedb71f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v)
> set_bit(KVM_REQ_CLOCK_SYNC, &v->requests);
> }
>
> +static inline int kvm_tsc_reliable(void)
> +{
> + return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
> + !check_tsc_unstable());
> +}
> +
> void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
> {
> struct kvm *kvm = vcpu->kvm;
> - u64 offset;
> + u64 offset, ns, elapsed;
>
> spin_lock(&kvm->arch.tsc_write_lock);
> offset = data - native_read_tsc();
> - kvm->arch.last_tsc_nsec = get_kernel_ns();
> + ns = get_kernel_ns();
> + elapsed = ns - kvm->arch.last_tsc_nsec;
> +
> + /*
> + * Special case: identical 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).
> + *
> + * In that case, for a reliable TSC, we can match TSC offsets,
> + * or make a best guest using kernel_ns value.
> + */
> + if (data == kvm->arch.last_tsc_write && elapsed < 5 * NSEC_PER_SEC) {
> + if (kvm_tsc_reliable()) {
> + offset = kvm->arch.last_tsc_offset;
> + pr_debug("kvm: matched tsc offset for %llu\n", data);
> + } else {
> + u64 tsc_delta = elapsed * __get_cpu_var(cpu_tsc_khz);
> + tsc_delta = tsc_delta / USEC_PER_SEC;
> + offset -= tsc_delta;
> + pr_debug("kvm: adjusted tsc offset by %llu\n", tsc_delta);
> + }
> + ns = kvm->arch.last_tsc_nsec;
> + }
> + kvm->arch.last_tsc_nsec = ns;
> kvm->arch.last_tsc_write = data;
> kvm->arch.last_tsc_offset = offset;
> kvm_x86_ops->write_tsc_offset(vcpu, offset);
> --

Could extend this to handle migration.