Re: [Xen-devel] [PATCH 1/4] xen/events: Clear cpu_evtchn_mask before resuming

From: Boris Ostrovsky
Date: Wed Apr 29 2015 - 12:57:00 EST

On 04/29/2015 12:32 PM, David Vrabel wrote:
On 28/04/15 19:29, Boris Ostrovsky wrote:
On 04/28/2015 12:28 PM, David Vrabel wrote:
On 28/04/15 16:52, Boris Ostrovsky wrote:
When a guest is resumed, the hypervisor may change event channel
assignments. If this happens and the guest uses 2-level events it
is possible for the interrupt to be claimed by wrong VCPU since
cpu_evtchn_mask bits may be stale. This can happen even though
evtchn_2l_bind_to_cpu() attempts to clear old bits: irq_info that
is passed in is not necessarily the original one (from pre-migration
times) but instead is freshly allocated during resume and so any
information about which CPU the channel was bound to is lost.

Thus we should clear the mask during resume.

We also need to make sure that bits for xenstore and console channels
are set when these two subsystems are resumed. While rebind_evtchn_irq()
(which is invoked for both of them on a resume) calls
the latter will in fact postpone setting affinity until handling the
interrupt. But because cpu_evtchn_mask will have bits for these two
we won't be able to take the interrupt.

Setting IRQ_MOVE_PCNTXT flag for the two irqs avoids this problem by
allowing to set affinity immediately, which is safe for
--- a/drivers/tty/hvc/hvc_xen.c
+++ b/drivers/tty/hvc/hvc_xen.c
@@ -533,6 +533,7 @@ static int __init xen_hvc_init(void)
info = vtermno_to_xencons(HVC_COOKIE);
info->irq = bind_evtchn_to_irq(info->evtchn);
+ irq_set_status_flags(info->irq, IRQ_MOVE_PCNTXT);
if (info->irq < 0)
info->irq = 0; /* NO_IRQ */
diff --git a/drivers/xen/events/events_2l.c
index 5db43fc..7dd4631 100644
--- a/drivers/xen/events/events_2l.c
+++ b/drivers/xen/events/events_2l.c
@@ -345,6 +345,15 @@ irqreturn_t xen_debug_interrupt(int irq, void
+static void evtchn_2l_resume(void)
+ int i;
+ for_each_online_cpu(i)
+ memset(per_cpu(cpu_evtchn_mask, i), 0, sizeof(xen_ulong_t) *
static const struct evtchn_ops evtchn_ops_2l = {
.max_channels = evtchn_2l_max_channels,
.nr_channels = evtchn_2l_max_channels,
@@ -356,6 +365,7 @@ static const struct evtchn_ops evtchn_ops_2l = {
.mask = evtchn_2l_mask,
.unmask = evtchn_2l_unmask,
.handle_events = evtchn_2l_handle_events,
+ .resume = evtchn_2l_resume,
void __init xen_evtchn_2l_init(void)
diff --git a/drivers/xen/xenbus/xenbus_comms.c
index fdb0f33..30203d1 100644
--- a/drivers/xen/xenbus/xenbus_comms.c
+++ b/drivers/xen/xenbus/xenbus_comms.c
@@ -231,6 +231,7 @@ int xb_init_comms(void)
xenbus_irq = err;
+ irq_set_status_flags(xenbus_irq, IRQ_MOVE_PCNTXT);
IRQ_MOVE_PCNTXT means "Interrupt can be migrated from process context"
which doesn't really sound relevant to me here?

Thomas Glexnier is really not happy with mis-use of IRQ APIs.
Yes, this is somewhat expanding definition of IRQ_MOVE_PCNTXT but then
there are other devices that do the same (HPET, for one).
Ok. But why set it on only these two event channels and not every one?

On resume in xen_irq_resume() we do

for_each_possible_cpu(cpu) {

And console and xenstore are neither of the those three types. rebind_irq_to_cpu() essentially plays the role of these restore_*() routines, except it does not make the hypercall to let the hypervisor assign notify_vcpu_id and it doesn't do explicit bind_evtchn_to_cpu().

Which makes me think that perhaps we should modify rebind_irq_to_cpu() to in fact do these two things. We may still need to keep irq_set_affinity() so that generic affinity data is properly set up but this will be deferred until the first interrupt.

From the commit log the evtchn_2l_resume() fucntion that's added sounds
like it fixes the problem on its own?
It in fact makes this problem worse since now that cpu_evtchn_mask is
cleared during resume we cannot process the interrupt anymore in
evtchn_2l_handle_events(): irqs have to be bound to a cpu in order for
an interrupt to be processed.
Perhaps evtchn_2l_resume() should set the local cpu mask for any bound
event channels? And then you wouldn't need IRQ_MOVE_PCNTX.

But then (at least in 2-level case) more than one VCPUs may pick the same interrupt, won't they? Because the local cpu mask is what tells a VCPU that it is allowed to claim an interrupt.


We already have this issue even without clearing the mask if, say,
xenstore channel changes or if it gets bound to cpu other than 0 before
suspend. It's just that we rarely (if ever) see this happen.

We can explicitly call events_base.c:set_affinity_irq() from
rebind_evtchn_irq() after irq_set_affinity() if we want to avoid using
IRQ_MOVE_PCNTXT flag but that will also looks kind of strange.

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