Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

[Sean Christopherson <seanjc@google.com>]

On Sat, Sep 30, 2023, David Woodhouse wrote:
> @@ -146,6 +160,14 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
>  
>  static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, s64 delta_ns)
>  {
> +	/*
> +	 * Avoid races with the old timer firing. Checking timer_expires
> +	 * to avoid calling hrtimer_cancel() will only have false positives
> +	 * so is fine.
> +	 */
> +	if (vcpu->arch.xen.timer_expires)
> +		hrtimer_cancel(&vcpu->arch.xen.timer);
> +
>  	atomic_set(&vcpu->arch.xen.timer_pending, 0);
>  	vcpu->arch.xen.timer_expires = guest_abs;
>  
> @@ -1019,9 +1041,36 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  		break;
>  
>  	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> +		/*
> +		 * Ensure a consistent snapshot of state is captured, with a
> +		 * timer either being pending, or the event channel delivered
> +		 * to the corresponding bit in the shared_info. Not still
> +		 * lurking in the timer_pending flag for deferred delivery.
> +		 * Purely as an optimisation, if the timer_expires field is
> +		 * zero, that means the timer isn't active (or even in the
> +		 * timer_pending flag) and there is no need to cancel it.
> +		 */

Ah, kvm_xen_start_timer() zeros timer_pending.

Given that, shouldn't it be impossible for xen_timer_callback() to observe a
non-zero timer_pending value?  E.g. couldn't this code WARN?

	if (atomic_read(&vcpu->arch.xen.timer_pending))
		return HRTIMER_NORESTART;

Obviously not a blocker for this patch, I'm mostly just curious to know if I'm
missing something.