Re: [PATCH 05/17] Keep SMP VMs more in sync on unstable TSC

Avi Kivity
2010-06-15T08:11:56Z

On 06/15/2010 10:34 AM, Zachary Amsden wrote:
> SMP VMs on machines with unstable TSC have their TSC offset adjusted by the
> local offset delta from last measurement. This does not take into account how
> long it has been since the measurement, leading to drift. Minimize the drift
> by accounting for any time difference the kernel has observed.
>
> Signed-off-by: Zachary Amsden<zamsden@redhat.com>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/x86.c | 20 +++++++++++++++++++-
> 2 files changed, 20 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 94f6ce8..1afecd7 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -337,6 +337,7 @@ struct kvm_vcpu_arch {
> unsigned int time_offset;
> struct page *time_page;
> u64 last_host_tsc;
> + u64 last_host_ns;
>
> bool nmi_pending;
> bool nmi_injected;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 618c435..b1bdf05 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1810,6 +1810,19 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> /* Make sure TSC doesn't go backwards */
> s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
> native_read_tsc() - vcpu->arch.last_host_tsc;
> +
> + /* Subtract elapsed cycle time from the delta computation */
> + if (check_tsc_unstable()&& vcpu->arch.last_host_ns) {
> + s64 delta;
> + struct timespec ts;
> + ktime_get_ts(&ts);
> + monotonic_to_bootbased(&ts);
> + delta = timespec_to_ns(&ts) - vcpu->arch.last_host_ns;
> + delta = delta * per_cpu(cpu_tsc_khz, cpu);
> + delta = delta / USEC_PER_SEC;
> + tsc_delta -= delta;
> + }
> +
> if (tsc_delta< 0 || check_tsc_unstable())
> kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
> kvm_migrate_timers(vcpu);
> @@ -1832,8 +1845,13 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> * vcpu->cpu != cpu can not detect this condition. So set
> * vcpu->cpu = -1 to force the recalibration above.
> */
> - if (check_tsc_unstable())
> + if (check_tsc_unstable()) {
> + struct timespec ts;
> + ktime_get_ts(&ts);
> + monotonic_to_bootbased(&ts);
> + vcpu->arch.last_host_ns = timespec_to_ns(&ts);
> vcpu->cpu = -1;
> + }
> }
>

Is there no way to do this calculation entirely with ktime_ts? struct
timespec has nasty multiplies and divides.

--
error compiling committee.c: too many arguments to function