Re: [PATCH v1] KVM: arm64: vgic-v4: Don't forward an LPI with no target vCPU

[Marc Zyngier <maz@kernel.org>]

On Mon, 12 Oct 2026 15:13:51 +0100,
Fuad Tabba <fuad.tabba@linux.dev> wrote:
> 
> Hi Marc,
> 
> On Mon, 12 Oct 2026 12:18:51 +0100, Marc Zyngier <maz@kernel.org> wrote:
> [...]
> > Isn't the core problem that we don't correctly handle the UNPRED
> > double mapping of the same LPI? I'd rather we attack the issue at the
> > source instead of papering over the consequences.
> >
> > I hacked the following, which is obviously untested. Can you give it a
> > go?
> 
> With your diff on rc3, the second MAPTI is dropped, and forwarding the
> vfio-pci MSI to its event leaves the host up.
> 
> However, a vgic_irq outlives its ITE while the LPI is pending on a
> vCPU, so the check also drops a legitimate remap: a guest that
> DISCARDs a pending LPI and then maps it again loses the new mapping.

But this is yet another bug: 5.3.4 is pretty clear that DISCARD should
get rid of everything (pending bit and mapping). I reckon this is yet
another case of the initial implementation being overly worried about
kicking vcpus out of the guest when destructive action is required.

We should fix this.

Thanks,

	M.

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