Re: [KVM Clock Synchronization 2/4] Keep TSC synchronized across host suspend

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Dec 28, 2010 at 07:38:18PM -1000, Zachary Amsden wrote:
> During a host suspend, TSC may go backwards, which KVM interprets
> as an unstable TSC.  Technically, KVM should not be marking the
> TSC unstable, which causes the TSC clocksource to go bad, but
> should be adjusting the TSC offsets in such a case.
> 
> Signed-off-by: Zachary Amsden <zamsden@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h |    1 +
>  arch/x86/kvm/x86.c              |   66 +++++++++++++++++++++++++++++++++++---
>  2 files changed, 61 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 9e6fe39..63a82b0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -386,6 +386,7 @@ struct kvm_vcpu_arch {
>  	u64 last_kernel_ns;
>  	u64 last_tsc_nsec;
>  	u64 last_tsc_write;
> +	u64 tsc_offset_adjustment;
>  	bool tsc_catchup;
>  
>  	bool nmi_pending;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b9118f4..b509c01 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2042,12 +2042,20 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	}
>  
>  	kvm_x86_ops->vcpu_load(vcpu, cpu);
> +
> +	/* Apply any externally detected TSC adjustments (due to suspend) */
> +	if (unlikely(vcpu->arch.tsc_offset_adjustment)) {
> +		kvm_x86_ops->adjust_tsc_offset(vcpu,
> +			vcpu->arch.tsc_offset_adjustment);
> +		vcpu->arch.tsc_offset_adjustment = 0;
> +		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> +	}
>  	if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
>  		/* Make sure TSC doesn't go backwards */
>  		s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
>  				native_read_tsc() - vcpu->arch.last_host_tsc;
>  		if (tsc_delta < 0)
> -			mark_tsc_unstable("KVM discovered backwards TSC");
> +			WARN_ON(!check_tsc_unstable());
>  		if (check_tsc_unstable()) {
>  			kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
>  			vcpu->arch.tsc_catchup = 1;
> @@ -5778,14 +5786,60 @@ int kvm_arch_hardware_enable(void *garbage)
>  {
>  	struct kvm *kvm;
>  	struct kvm_vcpu *vcpu;
> -	int i;
> +	int i, ret;
> +	u64 local_tsc;
> +	u64 max_tsc = 0;
> +	bool stable, backwards_tsc = false;
>  
>  	kvm_shared_msr_cpu_online();
> -	list_for_each_entry(kvm, &vm_list, vm_list)
> -		kvm_for_each_vcpu(i, vcpu, kvm)
> -			if (vcpu->cpu == smp_processor_id())
> +	local_tsc = native_read_tsc();
> +	stable = !check_tsc_unstable();
> +	ret = kvm_x86_ops->hardware_enable(garbage);
> +	if (ret)
> +		return ret;
> +
> +	list_for_each_entry(kvm, &vm_list, vm_list) {
> +		kvm_for_each_vcpu(i, vcpu, kvm) {
> +			if (!stable && vcpu->cpu == smp_processor_id())
>  				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> -	return kvm_x86_ops->hardware_enable(garbage);
> +			if (stable && vcpu->arch.last_host_tsc > local_tsc) {
> +				backwards_tsc = true;
> +				if (vcpu->arch.last_host_tsc > max_tsc)
> +					max_tsc = vcpu->arch.last_host_tsc;
> +			}
> +		}
> +	}
> +
> +	/*
> +	 * Sometimes, reliable TSCs go backwards.  This happens
> +	 * on platforms that reset TSC during suspend or hibernate
> +	 * actions, but maintain synchronization.  We must compensate.
> +	 * Unfortunately, we can't bring the TSCs fully up to date
> +	 * with real time.  The reason is that we aren't yet far
> +	 * enough into CPU bringup that we know how much real time
> +	 * has actually elapsed; our helper function, get_kernel_ns()
> +	 * will be using boot variables that haven't been updated yet.
> +	 * We simply find the maximum observed TSC above, then record
> +	 * the adjustment to TSC in each VCPU.  When the VCPU later
> +	 * gets loaded, the adjustment will be applied.  Note that we
> +	 * accumulate adjustments, in case multiple suspend cycles
> +	 * happen before the VCPU gets a chance to run again.
> +	 *
> +	 * Note that unreliable TSCs will be compensated already by
> +	 * the logic in vcpu_load, which sets the TSC to catchup mode.
> +	 * This will catchup all VCPUs to real time, but cannot
> +	 * guarantee synchronization.
> +	 */
> +	if (backwards_tsc) {
> +		u64 delta_cyc = max_tsc - local_tsc;
> +		list_for_each_entry(kvm, &vm_list, vm_list)
> +			kvm_for_each_vcpu(i, vcpu, kvm) {
> +				vcpu->arch.tsc_offset_adjustment += delta_cyc;
> +				vcpu->arch.last_host_tsc = 0;
> +			}
> +	}
> +
> +	return 0;
>  }

Wouldnt it be simpler to use cyc2ns_offset (or something equivalent)? In
any case, you forgot to compare smp_processor_id.