Re: [PATCH] KVM: arm/arm64: vgic-its: Fix restoration of unmapped collections

[Zenghui Yu]

Hi Marc,

On 2019/12/13 19:28, Marc Zyngier wrote:
> Hi Zenghui,
>
> On 2019-12-13 10:53, Zenghui Yu wrote:
> > Hi Eric,
> >
> > On 2019/12/13 17:42, Eric Auger wrote:
> > > Saving/restoring an unmapped collection is a valid scenario. For
> > > example this happens if a MAPTI command was sent, featuring an
> > > unmapped collection. At the moment the CTE fails to be restored.
> > > Only compare against the number of online vcpus if the rdist
> > > base is set.
> >
> > Have you actually seen a problem and this patch fixed it? To be honest,
> > I'm surprised to find that we can map a LPI to an unmapped collection ;)
> > (and prevent it to be delivered to vcpu with an INT_UNMAPPED_INTERRUPT
> > error, until someone had actually mapped the collection).
> > After a quick glance of spec (MAPTI), just as you said, this is valid.
>
> Yes, this is one of the (many) odd bits in the architecture. And there is
> a bizarre wording in the MAPC description when V=0:
>
> "Behavior is unpredictable if there are interrupts that are mapped to the
> specified collection, with the restriction that further translation 
> requests
> from that device are ignored."
>
> It is really odd that:
>
> - it is unpredictable to unmap the collection with mapped interrupts,
>  Â but mapping interrupts to an unmapped collection is fine

Yes, looks odd... Without Eric's patch, I won't even notice it.

I guess that unmapping the collection with mapped interrupts will make
it difficult for Hardware to manage those interrupts' internal states,
but only a guess.

> - the notion of "interrupts from that device" doesn't match any of the
>  Â MAPC parameters

Looks like a writing mistake, a better statement *might be*
"further translation requests targeting that ICID are ignored"?

> Do you hate the GIC already? ;-)

Not yet! (I'd like to continue being messed with GIC and see when I
will hate it :)

> > If Marc has no objection to this fix, please add
> >
> > Reviewed-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
>
> Thanks for that, I've applied it to the patch and will push out
> the update as soon as ra.kernel.org is reachable again.

Thanks,
Zenghui