Re: [PATCH 03/18] TSC reset compensation

Avi Kivity
2010-07-18T14:35:01Z

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.

> kvm_x86_ops->write_tsc_offset(vcpu, offset);
> spin_unlock(&kvm->arch.tsc_write_lock);
>
>

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