linux-kernel - Re: [PATCH v4 08/20] irqchip/gic-v4.1: Plumb get/set_irqchip

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Date:   Mon, 2 Mar 2020 16:18:07 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jason Cooper <jason@...edaemon.net>,
        "Robert Richter" <rrichter@...vell.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric Auger" <eric.auger@...hat.com>,
        James Morse <james.morse@....com>,
        "Julien Thierry" <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH v4 08/20] irqchip/gic-v4.1: Plumb get/set_irqchip_state
 SGI callbacks

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.


Thanks,
Zenghui