Re: [Xen-devel] [PATCH v6 2/5] xen/arm: inflight irqs during migration

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 24 Jun 2014, Julien Grall wrote:
> On 24/06/14 12:57, Stefano Stabellini wrote:
> > On Mon, 23 Jun 2014, Julien Grall wrote:
> > > Hi Stefano,
> > > 
> > > On 23/06/14 17:37, Stefano Stabellini wrote:
> > > > @@ -786,9 +837,14 @@ void vgic_vcpu_inject_irq(struct vcpu *v, unsigned
> > > > int irq)
> > > > 
> > > >        spin_lock_irqsave(&v->arch.vgic.lock, flags);
> > > > 
> > > > +    set_bit(GIC_IRQ_GUEST_QUEUED, &n->status);
> > > > +    /* update QUEUED before MIGRATING */
> > > > +    smp_wmb();
> > > > +    if ( test_bit(GIC_IRQ_GUEST_MIGRATING, &n->status) )
> > > > +        goto out;
> > > 
> > > Why do you kick the current VCPU here? It looks like useless because the
> > > migration will take care of it.
> > 
> > With the physical follow virtual patch, the vcpu_unblock below is going
> > to be mostly useless but harmless as vgic_vcpu_inject_irq is going to be
> > called on
> > the pcpu running the destination vcpu. smp_send_event_check_mask won't be
> > called as
> > v == current.
> > 
> > On the other hand without that patch, the pcpu receiving the physical
> > interrupt could be different from any of the pcpus running the vcpus
> > involved in vcpu migration, therefore we would need the kick to wake up
> > the destination vcpu.
> 
> If the MIGRATING bit is set it means that an IRQ is already inflight, and
> therefore gic_update_one_lr will take care of injecting this IRQ to the new
> VCPU by calling vgic_vcpu_inject_irq.
> So kicking the new VCPU here is pointless and may unschedule another VCPU for
> nothing. The latter may be able to run a bit more.
> 
> With your patch #4 (physical IRQ follow virtual IRQ), there is a tiny range
> the VCPU may not run on the same pCPU where the physical IRQ as been routed.
> This is the case when the VCPU is unscheduled.

I think that you are right. I'll remove the vcpu kick.
Also I realize now that I might be able to simplify the code by updating
the affinity of the physical irq only after the MIGRATING virq has been
EOIed.

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