Re: [KVM timekeeping 25/35] Add clock catchup mode

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Aug 19, 2010 at 10:07:39PM -1000, Zachary Amsden wrote:
> Make the clock update handler handle generic clock synchronization,
> not just KVM clock.  We add a catchup mode which keeps passthrough
> TSC in line with absolute guest TSC.
> 
> Signed-off-by: Zachary Amsden <zamsden@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h |    1 +
>  arch/x86/kvm/x86.c              |   55 ++++++++++++++++++++++++++------------
>  2 files changed, 38 insertions(+), 18 deletions(-)
> 

>  	kvm_x86_ops->vcpu_load(vcpu, cpu);
> -	if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
> +	if (unlikely(vcpu->cpu != cpu) || vcpu->arch.tsc_rebase) {
>  		/* 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");
> -		if (check_tsc_unstable())
> +		if (check_tsc_unstable()) {
>  			kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
> -		kvm_migrate_timers(vcpu);
> +			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> +		}
> +		if (vcpu->cpu != cpu)
> +			kvm_migrate_timers(vcpu);
>  		vcpu->cpu = cpu;
> +		vcpu->arch.tsc_rebase = 0;
>  	}
>  }
>  
> @@ -1947,6 +1961,12 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  	kvm_x86_ops->vcpu_put(vcpu);
>  	kvm_put_guest_fpu(vcpu);
>  	vcpu->arch.last_host_tsc = native_read_tsc();
> +
> +	/* For unstable TSC, force compensation and catchup on next CPU */
> +	if (check_tsc_unstable()) {
> +		vcpu->arch.tsc_rebase = 1;
> +		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> +	}

The mix between catchup,trap versus stable,unstable TSC is confusing and
difficult to grasp. Can you please introduce all the infrastructure
first, then control usage of them in centralized places? Examples:

+static void kvm_update_tsc_trapping(struct kvm *kvm)
+{
+       int trap, i;
+       struct kvm_vcpu *vcpu;
+
+       trap = check_tsc_unstable() && atomic_read(&kvm->online_vcpus) > 1;
+       kvm_for_each_vcpu(i, vcpu, kvm)
+               kvm_x86_ops->set_tsc_trap(vcpu, trap && !vcpu->arch.time_page);
+}

+       /* For unstable TSC, force compensation and catchup on next CPU */
+       if (check_tsc_unstable()) {
+               vcpu->arch.tsc_rebase = 1;
+               kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+       }


kvm_guest_time_update is becoming very confusing too. I understand this
is due to the many cases its dealing with, but please make it as simple
as possible.

+       /*
+        * If we are trapping and no longer need to, use catchup to
+        * ensure passthrough TSC will not be less than trapped TSC
+        */
+       if (vcpu->tsc_mode == TSC_MODE_PASSTHROUGH && vcpu->tsc_trapping &&
+           ((this_tsc_khz <= v->kvm->arch.virtual_tsc_khz || kvmclock))) {
+               catchup = 1;

What, TSC trapping with kvmclock enabled?

For both catchup and trapping the resolution of the host clock is
important, as Glauber commented for kvmclock. Can you comment on the
problems that arrive from a low res clock for both modes?

Similarly for catchup mode, the effect of exit frequency. No need for
any guarantees?