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

[Boris Ostrovsky]

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 irq_set_affinity(),
> > 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
> > xen_irq_info_evtchn_setup().)
> [...]
> > @@ -1279,8 +1280,16 @@ void rebind_evtchn_irq(int evtchn, int irq)
> >   
> >   	mutex_unlock(&irq_mapping_update_lock);
> >   
> > -	/* 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.

That's why I also replaced cpumask_of(0) with cpumask_of(info->cpu) in 
irq_set_affinity() call.

-boris


> If you agree I can remove the hypercall and apply this series.
>
> > +	else
> > +		pr_warn("Failed binding port %d to cpu %d\n",
> > +			evtchn, info->cpu);
> > +
> > +	/* This will be deferred until interrupt is processed */
> > +	irq_set_affinity(irq, cpumask_of(info->cpu));
> David

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