linux-kernel - Re: [PATCH] irqchip/gic-v4.1: Disallow setting affinity for virtual SGIs

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Date:   Tue, 14 Apr 2020 16:09:35 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <tglx@...utronix.de>,
        <jason@...edaemon.net>, <wanghaibin.wang@...wei.com>,
        Nianyao Tang <tangnianyao@...wei.com>
Subject: Re: [PATCH] irqchip/gic-v4.1: Disallow setting affinity for virtual
 SGIs

Hi Marc,

On 2020/4/11 17:41, Marc Zyngier wrote:
> Hi Zenghui,
> 
> On Sat, 11 Apr 2020 10:10:32 +0100,
> Zenghui Yu <yuzenghui@...wei.com> wrote:
>>
>> Running a guest on the GICv4.1-implemented board, we will get the
>> following warning:
>>
>> [   59.062120] genirq: irq_chip GICv4.1-sgi did not update eff. affinity mask of irq 46
>>
>> It may be caused by irqbalance (or other userspace tools) which tries to
>> change the affinity of virtual SGIs on the host. One way to "fix" it is
>> to update the effective_affinity value in irq_set_affinity callback. But
>> as the comment above says, "There is no notion of affinity for virtual
>> SGIs, at least not on the host", doing so only makes things confusing.
>>
>> Given the vSGIs are private to the specified vPE, changing the affinity
>> on host is actually meaningless and achieves nothing. Let's just forbid
>> it.
>>
>> Reported-by: Nianyao Tang <tangnianyao@...wei.com>
>> Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>
>> ---
>>
>> Hi Marc,
>>
>> This just restores the behavior of your v5 [*]. I wonder that what's the
>> reason to change it to 'return IRQ_SET_MASK_OK' in v6? What I've missed
>> here?
>>
>> [*] https://lore.kernel.org/kvm/20200304203330.4967-9-maz@kernel.org/
> 
> Not allowing the affinity move results in the kernel screaming when
> playing with CPU hotplug (it really wants to move the interrupt
> around). Which is why I dropped the -EINVAL, therefore introducing
> another bug. I fixed it with this patch[1], which I was planning to
> post after -rc1.

I didn't realize the CPU hotplug case. Please take your approach to
fix it. (As mentioned, this was also one way I planned to fix it.)

> 
> Let me know what you think

TBH, I'm not very familiar with the IRQ core behavior on CPU hotplug.
I will read further and comment on your formal patch (please cc me),
but now spinning on some other things...


Thanks,
Zenghui