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Subject: Re: [RFC PATCH] perf/smmuv3: Fix shared interrupt handling
To:     John Garry <john.garry@huawei.com>,
        "will@kernel.org" <will@kernel.org>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>
Cc:     "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@huawei.com>,
        "harb@amperecomputing.com" <harb@amperecomputing.com>,
        "tuanphan@os.amperecomputing.com" <tuanphan@os.amperecomputing.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <d73dd8c3579fbf713d6215317404549aede8ad2d.1586363449.git.robin.murphy@arm.com>
 <34dd7c2e-b6db-684f-f0a2-73f2e6951308@huawei.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <6bd103f2-1034-60f0-53a3-17162400a452@arm.com>
Date:   Thu, 9 Apr 2020 10:54:11 +0100
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On 2020-04-09 8:02 am, John Garry wrote:
> On 08/04/2020 17:49, Robin Murphy wrote:
>> IRQF_SHARED is dangerous, since it allows other agents to retarget the
>> IRQ's affinity without migrating PMU contexts to match, breaking the way
>> in which perf manages mutual exclusion for accessing events. Although
>> this means it's not realistically possible to support PMU IRQs being
>> shared with other drivers, we *can* handle sharing between multiple PMU
>> instances with some explicit affinity bookkeeping and manual interrupt
>> multiplexing.
> 
> Hi Robin,
> 
> Out of curiosity, do we even need to support shared interrupts for any 
> implementations today?

Not that I know of, but we need the mitigation in general for future 
drivers[1], and since this one already had a suspicious IRQF_SHARED it 
was the ideal victim for prototyping. I haven't dared ask about Ampere's 
SMMU story... :)

> D06 board:
> 
> john@ubuntu:~$ more /proc/interrupts | grep smmuv3-pmu
> 
>   989:  0  0  0  0  ITS-pMSI 133120 Edge  smmuv3-pmu
>   990:  0  0  0  0  ITS-pMSI 135168 Edge  smmuv3-pmu
>   991:  0  0  0  0  ITS-pMSI 137216 Edge  smmuv3-pmu
>   992:  0  0  0  0  ITS-pMSI 139264 Edge  smmuv3-pmu
>   993:  0  0  0  0  ITS-pMSI 141312 Edge  smmuv3-pmu
>   994:  0  0  0  0  ITS-pMSI 143360 Edge  smmuv3-pmu
>   995:  0  0  0  0  ITS-pMSI 145408 Edge  smmuv3-pmu
>   996:  0  0  0  0  ITS-pMSI 147456 Edge  smmuv3-pmu

Yeah, MSIs are the best way to defeat any interrupt wiring!

Robin.

[1] 
https://lore.kernel.org/linux-arm-kernel/3efa118a-5c85-6af9-e676-44087f1d398e@arm.com/