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Re: [Xen-devel] [PATCH v2 1/4] xen/events: Clear cpu_evtchn_mask before resuming

On 05/01/2015 11:25 AM, David Vrabel wrote:
On 01/05/15 14:39, Boris Ostrovsky wrote:
On 05/01/2015 06:46 AM, David Vrabel wrote:
On 29/04/15 22:10, 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
cleared we won't be able to take the interrupt.

With that in mind, we need to bind those two channels explicitly in
rebind_evtchn_irq(). We will keep irq_set_affinity() so that we have a
pass through generic irq affinity code later, in case something needs
to be updated there as well.

(Also replace cpumask_of(0) with cpumask_of(info->cpu) in
rebind_evtchn_irq(): it should be set to zero in preceding
@@ -1279,8 +1280,16 @@ void rebind_evtchn_irq(int evtchn, int irq)
   -    /* new event channels are always bound to cpu 0 */
-    irq_set_affinity(irq, cpumask_of(0));
+    bind_vcpu.port = evtchn;
+    bind_vcpu.vcpu = info->cpu;
+    if (HYPERVISOR_event_channel_op(EVTCHNOP_bind_vcpu, &bind_vcpu)
== 0)
+        bind_evtchn_to_cpu(evtchn, info->cpu);
Isn't the hypercall is unnecessary since this is a new event channel
it's already bound to VCPU 0 and info->cpu == 0?

I think only the bind_evtchn_to_cpu() call is needed here.

True. However, I added the hypercall here to make the routine
independent of what happens in other parts (hypervisor binding new
channels to cpu0, xen_irq_info_evtchn_setup() initializing to zero,
etc.). This way, if either of these two change in the future (unlikely,
but possible) this routine will still work as expected.
New event channels being bound to VCPU0 is part of the ABI and cannot
change, so I've taken the hypercall and its dodgy looking error path out.

I've applied this series to for-linus-4.1b but before I push, do any of
these need to be tagged for stable?

Yes, all of them need to. The first one can probably be easily backported starting from whenever FIFO events went in (3.14?) but the rest could go back all the way to 3.2.


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