linux-kernel - Re: [PATCH v7 5/7] KVM: arm64: pmu: Make overflow handler NMI safe

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Message-ID: <6b1ecdef-3a42-c428-2309-753b1470e3de@arm.com>
Date:   Wed, 30 Sep 2020 13:37:49 +0100
From:   Alexandru Elisei <alexandru.elisei@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, sumit.garg@...aro.org, maz@...nel.org,
        swboyd@...omium.org, catalin.marinas@....com,
        Julien Thierry <julien.thierry@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Will Deacon <will.deacon@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v7 5/7] KVM: arm64: pmu: Make overflow handler NMI safe

Hi Will,

On 9/28/20 6:57 PM, Will Deacon wrote:

> On Thu, Sep 24, 2020 at 12:07:04PM +0100, Alexandru Elisei wrote:
>> From: Julien Thierry <julien.thierry@....com>
>>
>> kvm_vcpu_kick() is not NMI safe. When the overflow handler is called from
>> NMI context, defer waking the vcpu to an irq_work queue.
>>
>> A vcpu can be freed while it's not running by kvm_destroy_vm(). Prevent
>> running the irq_work for a non-existent vcpu by calling irq_work_sync() on
>> the PMU destroy path.
>>
>> Cc: Julien Thierry <julien.thierry.kdev@...il.com>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: James Morse <james.morse@....com>
>> Cc: Suzuki K Pouloze <suzuki.poulose@....com>
>> Cc: kvm@...r.kernel.org
>> Cc: kvmarm@...ts.cs.columbia.edu
>> Signed-off-by: Julien Thierry <julien.thierry@....com>
>> Tested-by: Sumit Garg <sumit.garg@...aro.org> (Developerbox)
>> [Alexandru E.: Added irq_work_sync()]
>> Signed-off-by: Alexandru Elisei <alexandru.elisei@....com>
>> ---
>> I suggested in v6 that I will add an irq_work_sync() to
>> kvm_pmu_vcpu_reset(). It turns out it's not necessary: a vcpu reset is done
>> by the vcpu being reset with interrupts enabled, which means all the work
>> has had a chance to run before the reset takes place.
> I don't understand this ^^

Marc had the same comment, I replied in his email. I thought about it and you're
right, it doesn't make much sense.

>
> But the patch itself looks good, so I'm going to queue this lot anyway!

Thank you for picking up the series!

Thanks,
Alex