On Tue, Dec 28, 2010 at 07:38:20PM -1000, Zachary Amsden wrote:
> On systems with synchronized TSCs, we still have VCPU individual
> KVM clocks, each with their own computed offset. As this all happens
> at different times, the computed KVM clock offset can vary, causing a
> globally visible backwards clock. Currently this is protected against
> by using an atomic compare to ensure it does not happen.
>
> This change should remove that requirement.
>
> Signed-off-by: Zachary Amsden <zams...@redhat.com>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/x86.c | 42
> ++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 42 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 8d829b8..ff651b7 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -445,6 +445,7 @@ struct kvm_arch {
> unsigned long irq_sources_bitmap;
> s64 kvmclock_offset;
> spinlock_t clock_lock;
> + struct pvclock_vcpu_time_info master_clock;
> u64 last_tsc_nsec;
> u64 last_tsc_offset;
> u64 last_tsc_write;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 59d5999..a339e50 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1116,6 +1116,38 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> return 0;
>
> /*
> + * If there is a stable TSC, we use a master reference clock for
> + * the KVM clock; otherwise, individual computations for each VCPU
> + * would exhibit slight drift relative to each other, which could
> + * cause global time to go backwards.
> + *
> + * If the master clock has no TSC timestamp, that means we must
> + * recompute the clock as either some real time has elapsed during
> + * a suspend cycle, or we are measuring the clock for the first time
> + * during VM creation (or following a migration). Since master clock
> + * changes should happen only at rare occasions, so we can ignore
> + * the precautions below.
> + */
> + if (!check_tsc_unstable()) {
> + struct pvclock_vcpu_time_info *master =
> + &v->kvm->arch.master_clock;
> + if (vcpu->hv_clock.version != master->version) {
> + spin_lock(&v->kvm->arch.clock_lock);
> + WARN_ON(master->version < vcpu->hv_clock.version);
> + if (!master->tsc_timestamp) {
> + pr_debug("KVM: computing new master clock\n");
> + update_pvclock(v, master, tsc_timestamp,
> + kernel_ns, tsc_khz);
> + }
> + memcpy(&vcpu->hv_clock, master, sizeof(*master));
> + spin_unlock(&v->kvm->arch.clock_lock);
> + update_user_kvmclock(v, &vcpu->hv_clock);
> + } else
> + pr_debug("ignoring spurious KVM clock update");
> + return 0;
> + }
This assumes guest TSC is synchronized across vcpus... Is this always
true?
Also, for stable TSC hosts, kvmclock update is performed only on VM
creation / host resume these days... Can you describe the problem in
more detail?
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