Re: [PATCH v3 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending state to physical side

[Marc Zyngier <maz@kernel.org>]

On Fri, 12 Mar 2021 10:48:29 +0000,
Shenming Lu <lushenming@huawei.com> wrote:
> 
> On 2021/3/12 17:05, Marc Zyngier wrote:
> > On Thu, 11 Mar 2021 12:32:07 +0000,
> > Shenming Lu <lushenming@huawei.com> wrote:
> >>
> >> On 2021/3/11 17:14, Marc Zyngier wrote:
> >>> On Wed, 27 Jan 2021 12:13:36 +0000,
> >>> Shenming Lu <lushenming@huawei.com> wrote:
> >>>>
> >>>> From: Zenghui Yu <yuzenghui@huawei.com>
> >>>>
> >>>> When setting the forwarding path of a VLPI (switch to the HW mode),
> >>>> we could also transfer the pending state from irq->pending_latch to
> >>>> VPT (especially in migration, the pending states of VLPIs are restored
> >>>> into kvm’s vgic first). And we currently send "INT+VSYNC" to trigger
> >>>> a VLPI to pending.
> >>>>
> >>>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
> >>>> Signed-off-by: Shenming Lu <lushenming@huawei.com>
> >>>> ---
> >>>>  arch/arm64/kvm/vgic/vgic-v4.c | 14 ++++++++++++++
> >>>>  1 file changed, 14 insertions(+)
> >>>>
> >>>> diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
> >>>> index ac029ba3d337..a3542af6f04a 100644
> >>>> --- a/arch/arm64/kvm/vgic/vgic-v4.c
> >>>> +++ b/arch/arm64/kvm/vgic/vgic-v4.c
> >>>> @@ -449,6 +449,20 @@ int kvm_vgic_v4_set_forwarding(struct kvm *kvm, int virq,
> >>>>  	irq->host_irq	= virq;
> >>>>  	atomic_inc(&map.vpe->vlpi_count);
> >>>>  
> >>>> +	/* Transfer pending state */
> >>>> +	if (irq->pending_latch) {
> >>>> +		ret = irq_set_irqchip_state(irq->host_irq,
> >>>> +					    IRQCHIP_STATE_PENDING,
> >>>> +					    irq->pending_latch);
> >>>> +		WARN_RATELIMIT(ret, "IRQ %d", irq->host_irq);
> >>>> +
> >>>> +		/*
> >>>> +		 * Let it be pruned from ap_list later and don't bother
> >>>> +		 * the List Register.
> >>>> +		 */
> >>>> +		irq->pending_latch = false;
> >>>
> >>> NAK. If the interrupt is on the AP list, it must be pruned from it
> >>> *immediately*. The only case where it can be !pending and still on the
> >>> AP list is in interval between sync and prune. If we start messing
> >>> with this, we can't reason about the state of this list anymore.
> >>>
> >>> Consider calling vgic_queue_irq_unlock() here.
> >>
> >> Thanks for giving a hint, but it seems that vgic_queue_irq_unlock() only
> >> queues an IRQ after checking, did you mean vgic_prune_ap_list() instead?
> > 
> > No, I really mean vgic_queue_irq_unlock(). It can be used to remove
> > the pending state from an interrupt, and drop it from the AP
> > list. This is exactly what happens when clearing the pending state of
> > a level interrupt, for example.
> 
> Hi, I have gone through vgic_queue_irq_unlock more than once, but
> still can't find the place in it to drop an IRQ from the AP
> list... Did I miss something ?...  Or could you help to point it
> out? Thanks very much for this!

NO, you are right. I think this is a missing optimisation. Please call
the function anyway, as that's what is required to communicate a
change of state in general.

I'll have a think about it.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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