Re: [Xen-devel] [PATCH v8 09/13] xen/arm: second irq injection while the first irq is still inflight

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 22/05/14 18:39, Stefano Stabellini wrote:
> On Thu, 22 May 2014, Julien Grall wrote:
> > > while the first one is still active.
> > > If the first irq is already pending (not active), clear
> > > GIC_IRQ_GUEST_QUEUED because the guest doesn't need a second
> > > notification.If the irq has already been EOI'ed then just clear the
> > > GICH_LR right away and move the interrupt to lr_pending so that it is
> > > going to be reinjected by gic_restore_pending_irqs on return to guest.
> > >
> > > If the target cpu is not the current cpu, then set GIC_IRQ_GUEST_QUEUED
> > > and send an SGI. The target cpu is going to be interrupted and call
> > > gic_clear_lrs, that is going to take the same actions.
> > >
> > > Do not call vgic_vcpu_inject_irq from gic_inject if
> > > evtchn_upcall_pending is set. If we remove that call, we don't need to
> > > special case evtchn_irq in vgic_vcpu_inject_irq anymore.
> > > We need to force the first injection of evtchn_irq (call
> > > gic_vcpu_inject_irq) from vgic_enable_irqs because evtchn_upcall_pending
> > > is already set by common code on vcpu creation.
> > If you only need it for the first time. Why can't you call vgic_inject_irq
> > with the IRQ evtchn when the VCPU is turn on?
> >
> > This would remove every hack with this IRQ in the GIC code.
> In principle sounds nice, but in practice it is difficult and risks
> being racy. In vgic_vcpu_inject_irq we have:
>
>      /* vcpu offline */
>      if ( test_bit(_VPF_down, &v->pause_flags) )
>      {
>          spin_unlock_irqrestore(&v->arch.vgic.lock, flags);
>          return;
>      }
>
> So we can only inject the irq once the vcpu is properly up, that is
> certainly later than vcpu_initialise.
If we call vcpu_vgic_inject right before vcpu_wake (the _VPF_down flags 
has been cleared) we won't have any race condition.
This can be done in both arch/arm/vpsci.c and common/domain.c (VCPUOP_up).

It may require an arch specific function. Smth like arch_vcpu_prepare_up.

Regards,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel