Re: [PATCH v4 08/20] irqchip/gic-v4.1: Plumb get/set_irqchip_state SGI callbacks

[Marc Zyngier <maz@kernel.org>]

Hi Zenghui,

On 2020-03-02 08:18, Zenghui Yu wrote:
> On 2020/3/2 3:00, Marc Zyngier wrote:
>> On 2020-02-28 19:37, Marc Zyngier wrote:
>>> On 2020-02-20 03:11, Zenghui Yu wrote:
>> 
>>>> Do we really need to grab the vpe_lock for those which are belong to
>>>> the same irqchip with its_vpe_set_affinity()? The IRQ core code 
>>>> should
>>>> already ensure the mutual exclusion among them, wrong?
>>> 
>>> I've been trying to think about that, but jet-lag keeps getting in 
>>> the way.
>>> I empirically think that you are right, but I need to go and check 
>>> the various
>>> code paths to be sure. Hopefully I'll have a bit more brain space 
>>> next week.
>> 
>> So I slept on it and came back to my senses. The only case we actually 
>> need
>> to deal with is when an affinity change impacts *another* interrupt.
>> 
>> There is only two instances of this issue:
>> 
>> - vLPIs have their *physical* affinity impacted by the affinity of the
>>    vPE. Their virtual affinity is of course unchanged, but the 
>> physical
>>    one becomes important with direct invalidation. Taking a per-VPE 
>> lock
>>    in such context should address the issue.
>> 
>> - vSGIs have the exact same issue, plus the matter of requiring some
>>    *extra* one when reading the pending state, which requires a RMW
>>    on two different registers. This requires an extra per-RD lock.
> 
> Agreed with both!
> 
>> 
>> My original patch was stupidly complex, and the irq_desc lock is
>> perfectly enough to deal with anything that only affects the interrupt
>> state itself.
>> 
>> GICv4 + direct invalidation for vLPIs breaks this by bypassing the
>> serialization initially provided by the ITS, as the RD is completely
>> out of band. The per-vPE lock brings back this serialization.
>> 
>> I've updated the branch, which seems to run OK on D05. I still need
>> to run the usual tests on the FVP model though.
> 
> I have pulled the latest branch and it looks good to me, except for
> one remaining concern:
> 
> GICR_INV{LPI, ALL}R + GICR_SYNCR can also be accessed concurrently
> by multiple direct invalidation, should we also use the per-RD lock
> to ensure mutual exclusion?  It looks not so harmful though, as this
> will only increase one's polling time against the Busy bit (in my 
> view).
> 
> But I point it out again for confirmation.

I was about to say that it doesn't really matter because it is only a
performance optimisation (and we're noty quite there yet), until I 
spotted
this great nugget in the spec:

<quote>
Writing GICR_INVLPIR or GICR_INVALLR when GICR_SYNCR.Busy==1 is 
CONSTRAINED
UNPREDICTABLE:
- The write is IGNORED .
- The invalidate specified by the write is performed.
</quote>

So we really need some form of mutual exclusion on a per-RD basis to 
ensure
that no two invalidations occur at the same time, ensuring that Busy 
clears
between the two.

Thanks for the heads up,

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