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SubjectRe: [KVM timekeeping fixes 4/4] TSC catchup mode
On Sat, Sep 18, 2010 at 02:38:15PM -1000, Zachary Amsden wrote:
> Negate the effects of AN TYM spell while kvm thread is preempted by tracking
> conversion factor to the highest TSC rate and catching the TSC up when it has
> fallen behind the kernel view of time. Note that once triggered, we don't
> turn off catchup mode.
>
> A slightly more clever version of this is possible, which only does catchup
> when TSC rate drops, and which specifically targets only CPUs with broken
> TSC, but since these all are considered unstable_tsc(), this patch covers
> all necessary cases.
>
> Signed-off-by: Zachary Amsden <zamsden@redhat.com>
> ---
> arch/x86/include/asm/kvm_host.h | 6 +++
> arch/x86/kvm/x86.c | 87 +++++++++++++++++++++++++++++---------
> 2 files changed, 72 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 8c5779d..e209078 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -384,6 +384,9 @@ struct kvm_vcpu_arch {
> u64 last_host_tsc;
> u64 last_guest_tsc;
> u64 last_kernel_ns;
> + u64 last_tsc_nsec;
> + u64 last_tsc_write;
> + bool tsc_catchup;
>
> bool nmi_pending;
> bool nmi_injected;
> @@ -444,6 +447,9 @@ struct kvm_arch {
> u64 last_tsc_nsec;
> u64 last_tsc_offset;
> u64 last_tsc_write;
> + u32 virtual_tsc_khz;
> + u32 virtual_tsc_mult;
> + s8 virtual_tsc_shift;
>
> struct kvm_xen_hvm_config xen_hvm_config;
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 09f468a..9152156 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -962,6 +962,7 @@ static inline u64 get_kernel_ns(void)
> }
>
> static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
> +unsigned long max_tsc_khz;
>
> static inline int kvm_tsc_changes_freq(void)
> {
> @@ -985,6 +986,24 @@ static inline u64 nsec_to_cycles(u64 nsec)
> return ret;
> }
>
> +static void kvm_arch_set_tsc_khz(struct kvm *kvm, u32 this_tsc_khz)
> +{
> + /* Compute a scale to convert nanoseconds in TSC cycles */
> + kvm_get_time_scale(this_tsc_khz, NSEC_PER_SEC / 1000,
> + &kvm->arch.virtual_tsc_shift,
> + &kvm->arch.virtual_tsc_mult);
> + kvm->arch.virtual_tsc_khz = this_tsc_khz;
> +}
> +
> +static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
> +{
> + u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.last_tsc_nsec,
> + vcpu->kvm->arch.virtual_tsc_mult,
> + vcpu->kvm->arch.virtual_tsc_shift);
> + tsc += vcpu->arch.last_tsc_write;
> + return tsc;
> +}
> +
> void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
> {
> struct kvm *kvm = vcpu->kvm;
> @@ -1029,6 +1048,8 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
>
> /* Reset of TSC must disable overshoot protection below */
> vcpu->arch.hv_clock.tsc_timestamp = 0;
> + vcpu->arch.last_tsc_write = data;
> + vcpu->arch.last_tsc_nsec = ns;
> }
> EXPORT_SYMBOL_GPL(kvm_write_tsc);
>
> @@ -1041,22 +1062,42 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> s64 kernel_ns, max_kernel_ns;
> u64 tsc_timestamp;
>
> - if ((!vcpu->time_page))
> - return 0;
> -
> /* Keep irq disabled to prevent changes to the clock */
> local_irq_save(flags);
> kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
> kernel_ns = get_kernel_ns();
> this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
> - local_irq_restore(flags);
>
> if (unlikely(this_tsc_khz == 0)) {
> + local_irq_restore(flags);
> kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
> return 1;
> }
>
> /*
> + * We may have to catch up the TSC to match elapsed wall clock
> + * time for two reasons, even if kvmclock is used.
> + * 1) CPU could have been running below the maximum TSC rate

kvmclock handles frequency changes?

> + * 2) Broken TSC compensation resets the base at each VCPU
> + * entry to avoid unknown leaps of TSC even when running
> + * again on the same CPU. This may cause apparent elapsed
> + * time to disappear, and the guest to stand still or run
> + * very slowly.

I don't get this. Please explain.



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