Re: [PATCH 03/18] TSC reset compensation

[Zachary Amsden]

On 07/18/2010 04:34 AM, Avi Kivity wrote:
> On 07/13/2010 05:25 AM, 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.
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h 
> > b/arch/x86/include/asm/kvm_host.h
> > index 3b4efe2..4b42893 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -396,6 +396,9 @@ struct kvm_arch {
> >       unsigned long irq_sources_bitmap;
> >       s64 kvmclock_offset;
> >       spinlock_t tsc_write_lock;
> > +    u64 last_tsc_nsec;
> > +    u64 last_tsc_offset;
> > +    u64 last_tsc_write;
>
> So that we know what the lock protects, let's have
>
>    struct kvm_global_tsc {
>          spinlock_t lock;
>          ...
>     } tsc;
>
> > @@ -896,10 +896,39 @@ static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
> >   void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
> >   {
> >       struct kvm *kvm = vcpu->kvm;
> > -    u64 offset;
> > +    u64 offset, ns, elapsed;
> > +    struct timespec ts;
> >
> >       spin_lock(&kvm->arch.tsc_write_lock);
> >       offset = data - native_read_tsc();
> > +    ktime_get_ts(&ts);
> > +    monotonic_to_bootbased(&ts);
> > +    ns = timespec_to_ns(&ts);
> > +    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<  5ULL * 
> > NSEC_PER_SEC) {
> > +        if (!check_tsc_unstable()) {
> > +            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;
>
> We'd have a false alarm here during a reset within 5 seconds of boot.  
> Does it matter?  Easy to work around by forgetting the state during 
> reset.

Not forgetting, but ignoring; reset within 5 seconds will not reset TSC, 
which normally is fine.  The problem is that one CPU could reset within 
5 seconds and one slightly after.  Forgetting during reset is a good 
solution.
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