On 2020-12-31 11:58, Shenming Lu wrote:

On 2020/12/31 16:57, Marc Zyngier wrote:

quoted

Hi Shemming,

On 2020-12-31 06:28, Shenming Lu wrote:

quoted

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.

quoted

quoted

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...

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