Re: [PATCH v4 05/12] KVM: x86/xen: Don't lose the timer event when delivery fails transiently

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] KVM: x86/xen: xen_timer_callback() zeroes timer_expires on -EINVAL
--

commit 209a694ed87ce18b640accf23332c30c7e64aa39
Author: David Woodhouse <dwmw@amazon.co.uk>

KVM: x86/xen: Don't lose the timer event when delivery fails transiently

This patch defers Xen PV timer event delivery when kvm_xen_set_evtchn()
fails transiently (-EAGAIN), preserves timer_expires while clearing
timer_pending on -EINVAL, and updates KVM_XEN_VCPU_GET_ATTR(TIMER) to
avoid racing with pending retries or losing owed timer events.

> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 8c49ec8aa4d97..8a9d6963fc05a 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c

[ ... ]

> @@ -126,14 +139,47 @@ void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu)
>  {
>  	if (atomic_read(&vcpu->arch.xen.timer_pending) > 0) {
>  		struct kvm_xen_evtchn e;
> +		int rc;
>  
>  		e.vcpu_id = vcpu->vcpu_id;
>  		e.vcpu_idx = vcpu->vcpu_idx;
>  		e.port = vcpu->arch.xen.timer_virq;
>  		e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
>  
> -		kvm_xen_set_evtchn(&e, vcpu->kvm);
> +		rc = kvm_xen_set_evtchn(&e, vcpu->kvm);
> +
> +		/*
> +		 * Leave timer_pending set if the memslot is being updated
> +		 * (-EAGAIN), and raise KVM_REQ_UNBLOCK — as the timer
> +		 * callback does when it defers — so that the entry is
> +		 * aborted and this retried from vcpu_run(), rather than
> +		 * the guest running with the owed event undelivered until
> +		 * some unrelated VM exit.
> +		 */
> +		if (rc == -EAGAIN) {
> +			kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
> +			return;
> +		}
> +
> +		/*
> +		 * If the event cannot currently be delivered (-EINVAL:
> +		 * deactivated shared_info, or an out-of-range port), clear
> +		 * timer_pending — a halted vCPU would otherwise spin on
> +		 * kvm_cpu_has_pending_timer() — but keep timer_expires,
> +		 * which still reports the event as owed through
> +		 * KVM_XEN_VCPU_GET_ATTR; re-arming the timer with an expiry
> +		 * in the past fires it again.
> +		 */
> +		if (rc == -EINVAL) {
> +			atomic_set(&vcpu->arch.xen.timer_pending, 0);
> +			return;
> +		}
>  
> +		/*
> +		 * Delivered - or failed in a way which can never succeed
> +		 * (-EFAULT: a genuinely unbacked shared_info gfn). Either
> +		 * way the event is complete: drop it rather than spin.
> +		 */
>  		vcpu->arch.xen.timer_expires = 0;
>  		atomic_set(&vcpu->arch.xen.timer_pending, 0);
>  	}
> @@ -1315,17 +1361,21 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  		r = 0;
>  		break;
>  
> -	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> +	case KVM_XEN_VCPU_ATTR_TYPE_TIMER: {
> +		bool timer_armed = false;
> +
>  		/*
> -		 * 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.
> +		 * Ensure a consistent snapshot of state is captured: a timer
> +		 * which already fired has either been delivered to the
> +		 * corresponding bit in the shared_info, or is reported as
> +		 * still owed (expires_ns != 0, if the shared_info page was
> +		 * unavailable). 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.
>  		 */
>  		if (vcpu->arch.xen.timer_expires) {
> +			timer_armed = hrtimer_active(&vcpu->arch.xen.timer);
>  			hrtimer_cancel(&vcpu->arch.xen.timer);
>  			kvm_xen_inject_timer_irqs(vcpu);
>  		}
> @@ -1341,13 +1391,34 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  		 * That's fine, as the guest won't even have had a chance
>  		 * to run and handle the interrupt. Asserting an already
>  		 * pending event channel is idempotent.
> +		 *
> +		 * Don't re-arm if the flush above failed transiently and
> +		 * left the timer pending: delivery is owned by the retry
> +		 * on the guest entry path, and an immediate hrtimer fire
> +		 * would race it — the callback's timer_pending suppression
> +		 * check is not serialized against the entry path, so a
> +		 * late callback could re-assert a port the entry path had
> +		 * already delivered and the guest already consumed.
> +		 *
> +		 * An expiry already in the past fires immediately: the
> +		 * event is still owed.
> +		 *
> +		 * A timer still armed (not yet fired) across the flush is
> +		 * always restored, whatever its port. One which already
> +		 * fired is not re-fired if its port is out of range, where
> +		 * the immediate -EINVAL from the delivery path would destroy
> +		 * the timer_expires state just reported to userspace.
>  		 */
> -		if (vcpu->arch.xen.timer_expires)
> -			hrtimer_start_expires(&vcpu->arch.xen.timer,
> -					      HRTIMER_MODE_ABS_HARD);
> +		if (vcpu->arch.xen.timer_expires &&
> +		    !atomic_read(&vcpu->arch.xen.timer_pending) &&
> +		    (timer_armed ||
> +		     vcpu->arch.xen.timer_virq < kvm_max_evtchn_port(vcpu->kvm)))
> +			kvm_xen_start_timer(vcpu, vcpu->arch.xen.timer_expires,
> +					    false);

[Severity: Medium]
Should xen_timer_callback() also preserve vcpu->arch.xen.timer_expires
when kvm_xen_set_evtchn_fast() returns -EINVAL?

In kvm_xen_inject_timer_irqs(), -EINVAL from kvm_xen_set_evtchn() now
leaves vcpu->arch.xen.timer_expires intact while clearing timer_pending so
the owed timer state is preserved for KVM_XEN_VCPU_GET_ATTR.

However, when a timer fires with an out-of-range port (for example, a
64-bit port >= COMPAT_EVTCHN_2L_NR_CHANNELS before KVM_XEN_ATTR_TYPE_LONG_MODE
is configured), xen_timer_callback() calls kvm_xen_set_evtchn_fast() first:

arch/x86/kvm/xen.c:kvm_xen_set_evtchn_fast() {
	...
	if (xe->port >= max_evtchn_port(has_64bit_shinfo))
		return -EINVAL;

	rc = -EWOULDBLOCK;
	...
}

Because kvm_xen_set_evtchn_fast() returns -EINVAL rather than -EWOULDBLOCK,
xen_timer_callback() takes the rc != -EWOULDBLOCK branch and zeroes
vcpu->arch.xen.timer_expires immediately without deferring to
kvm_xen_inject_timer_irqs():

arch/x86/kvm/xen.c:xen_timer_callback() {
	...
	rc = kvm_xen_set_evtchn_fast(&e, vcpu->kvm);
	if (rc != -EWOULDBLOCK) {
		vcpu->arch.xen.timer_expires = 0;
		return HRTIMER_NORESTART;
	}
	...
}

As a result, any timer that fires with an out-of-range port (either when
the hrtimer expires, or synchronously via kvm_xen_start_timer() ->
xen_timer_callback() when delta <= 0, including in kvm_xen_vcpu_get_attr()
if timer_armed was true when hrtimer_active() ran and guest_now has reached
timer_expires by the time kvm_xen_start_timer() is called) will still lose
vcpu->arch.xen.timer_expires instead of keeping the event owed.

>  
>  		r = 0;
>  		break;
> +	}
>  
>  	case KVM_XEN_VCPU_ATTR_TYPE_UPCALL_VECTOR:
>  		data->u.vector = vcpu->arch.xen.upcall_vector;

[ ... ]

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-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261019162445.37219-1-dwmw2@infradead.org?part=5