Re: [PATCH RFC] KVM: arm64: vgic: Decouple the check of the EnableLPIs bit from the ITS LPI translation

[Marc Zyngier]

On 2020-12-31 11:58, Shenming Lu wrote:
> On 2020/12/31 16:57, Marc Zyngier wrote:
> > Hi Shemming,
> >
> > On 2020-12-31 06:28, Shenming Lu wrote:
> > > When the EnableLPIs bit is set to 0, any ITS LPI requests in the
> > > Redistributor would be ignored. And this check is independent from
> > > the ITS LPI translation. So it might be better to move the check
> > > of the EnableLPIs bit out of the LPI resolving, and also add it
> > > to the path that uses the translation cache.
> >
> > But by doing that, you are moving the overhead of checking for
> > EnableLPIs from the slow path (translation walk) to the fast
> > path (cache hit), which seems counter-productive.
>
> Oh, I didn't notice the overhead of the checking, I thought it would
> be negligible...

It probably doesn't show on a modern box, but some of the slower
systems might see it. Overall, this is a design decision to keep
the translation cache as simple and straightforward as possible:
if anything affects the output of the cache, we invalidate it,
and that's it.

> > > Besides it seems that
> > > by this the invalidating of the translation cache caused by the LPI
> > > disabling is unnecessary.
> > >
> > > Not sure if I have missed something... Thanks.
> >
> > I am certainly missing the purpose of this patch.
> >
> > The effect of EnableLPIs being zero is to drop the result of any
> > translation (a new pending bit) on the floor. Given that, it is
> > immaterial whether this causes a new translation or hits in the
> > cache, as the result is still to not pend a new interrupt.
> >
> > I get the feeling that you are trying to optimise for the unusual
> > case where EnableLPIs is 0 *and* you have a screaming device
> > injecting tons of interrupt. If that is the case, I don't think
> > this is worth it.
>
> In fact, I just found (imagining) that if the EnableLPIs bit is 0,
> the kvm_vgic_v4_set_forwarding() would fail when performing the LPI
> translation, but indeed we don't try to pend any interrupts there...
>
> By the way, it seems that the LPI disabling would not affect the
> injection of VLPIs...

Yes, good point. We could unmap the VPE from all ITS, which would result
in all translations to be discarded, but this has the really bad side
effect of *also* preventing the delivery of vSGIs, which isn't what
you'd expect.

Overall, I don't think there is a good way to support this, and maybe
we should just prevent EnableLPIs to be turned off when using direct
injection. After all, the architecture does allow that for GICv3
implementations, which is what we emulate.

Thanks,

        M.
--
Jazz is not dead. It just smells funny...