Re: [PATCH 3/5] KVM: arm64: Resolve vLPI by host IRQ in vgic_v4_unset_forwarding()

[Oliver Upton <oliver.upton@linux.dev>]

Hey,

On Fri, May 23, 2025 at 06:25:50PM +0100, Marc Zyngier wrote:
> > +int kvm_vgic_v4_unset_forwarding(struct kvm *kvm, int host_irq)
> >  {
> > -	struct vgic_its *its;
> >  	struct vgic_irq *irq;
> >  	unsigned long flags;
> >  	int ret = 0;
> > @@ -519,31 +536,19 @@ int kvm_vgic_v4_unset_forwarding(struct kvm *kvm, int virq,
> >  	if (!vgic_supports_direct_msis(kvm))
> >  		return 0;
> >  
> > -	/*
> > -	 * Get the ITS, and escape early on error (not a valid
> > -	 * doorbell for any of our vITSs).
> > -	 */
> > -	its = vgic_get_its(kvm, irq_entry);
> > -	if (IS_ERR(its))
> > +	irq = __vgic_host_irq_get_vlpi(kvm, host_irq);
> > +	if (!irq)
> >  		return 0;
> >  
> > -	mutex_lock(&its->its_lock);
> > -
> > -	ret = vgic_its_resolve_lpi(kvm, its, irq_entry->msi.devid,
> > -				   irq_entry->msi.data, &irq);
> > -	if (ret)
> > -		goto out;
> > -
> 
> Removing the reliance on the ITS locking is another thing that could
> be mentioned in the commit message, as it is slightly surprising to
> see it here, given that the previous patch is all about that.

Sure, I can add a bit of color. The prior patch left the its_lock in
place since it is still needed to walk the ITS device / ITE lists.

> My other gripe is that we lose the doorbell validation. I'm not sure
> it is a big deal, but I'd rather we keep verifying we're not being fed
> rubbish data, in case we need to rely on that in the future.

We still get doorbell validation on the set() side of things, which IMO
is where it belongs. Whether or not the MSI routing is garbage at time
of unset() isn't relevant, if we handed out a vLPI mapping we need to
kill it.

Thanks,
Oliver