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Subject: Re: [PROBLEM] Frequently get "irq 31: nobody cared" when passing
 through 2x GPUs that share same pci switch via vfio
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     linux-pci@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>,
        kvm@vger.kernel.org, nathan.langford@xcelesunifiedtechnologies.com
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 <20210914104301.48270518.alex.williamson@redhat.com>
 <9e8d0e9e-1d94-35e8-be1f-cf66916c24b2@canonical.com>
 <20210915103235.097202d2.alex.williamson@redhat.com>
From:   Matthew Ruffell <matthew.ruffell@canonical.com>
Message-ID: <4d9d0366-1769-691f-fcb0-3b14d468e36e@canonical.com>
Date:   Thu, 16 Sep 2021 17:13:21 +1200
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On 16/09/21 4:32 am, Alex Williamson wrote:
> On Wed, 15 Sep 2021 16:44:38 +1200
> Matthew Ruffell <matthew.ruffell@canonical.com> wrote:
>> On 15/09/21 4:43 am, Alex Williamson wrote:
>>>
>>> FWIW, I have access to a system with an NVIDIA K1 and M60, both use
>>> this same switch on-card and I've not experienced any issues assigning
>>> all the GPUs to a single VM.  Topo:
>>>
>>>  +-[0000:40]-+-02.0-[42-47]----00.0-[43-47]--+-08.0-[44]----00.0
>>>  |                                           +-09.0-[45]----00.0
>>>  |                                           +-10.0-[46]----00.0
>>>  |                                           \-11.0-[47]----00.0
>>>  \-[0000:00]-+-03.0-[04-07]----00.0-[05-07]--+-08.0-[06]----00.0
>>>                                              \-10.0-[07]----00.0
> 
> 
> I've actually found that the above configuration, assigning all 6 GPUs
> to a VM reproduces this pretty readily by simply rebooting the VM.  In
> my case, I don't have the panic-on-warn/oops that must be set on your
> kernel, so the result is far more benign, the IRQ gets masked until
> it's re-registered.
> 
> The fact that my upstream ports are using MSI seems irrelevant.

Hi Alex,


It is good news that you can reproduce an interrupt storm locally. Did a single

reboot trigger the storm, or did you have to loop the VM a few times?


On our system, if we don't have panic-on-warn/oops set, the system will

eventually grind to a halt and lock up, so we try to reset earlier on the first

oops, but we still get stuck in the crashkernel copying the IR tables from dmar.

> 
> Adding debugging to the vfio-pci interrupt handler, it's correctly
> deferring the interrupt as the GPU device is not identifying itself as
> the source of the interrupt via the status register.  In fact, setting
> the disable INTx bit in the GPU command register while the interrupt
> storm occurs does not stop the interrupts.
> 

Interesting. So the source of the interrupts could be from the PEX switch

itself?


We did a run with DisIntx+ set on the PEX switches, but it didn't make any

difference. Serial log showing DisIntx+ and full dmesg below:


https://paste.ubuntu.com/p/n3XshCxPT8/

> The interrupt storm does seem to be related to the bus resets, but I
> can't figure out yet how multiple devices per switch factors into the
> issue.  Serializing all bus resets via a mutex doesn't seem to change
> the behavior.

Very interesting indeed.

> I'm still investigating, but if anyone knows how to get access to the
> Broadcom datasheet or errata for this switch, please let me know.

I have tried reaching out to Broadcom asking for the datasheet and errata, but

I am unsure if they will get back to me.


They list the errata as publicly available on their website, in the 

Documentation > errata tab.

https://www.broadcom.com/products/pcie-switches-bridges/pcie-switches/pex8749#documentation


The file "PEX 8749/48/47/33/32/25/24/23/17/16/13/12 Errata" seems to be missing

though.

https://docs.broadcom.com/docs/PEX8749-48-47-33-32-25-24-23-17-16-13-12%20Errata-and-Cautions


An Intel document talks about the errata for the PEX 8749:

https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/rn/rn-ias-n3000-n.pdf

It links to the following URL, also missing.

https://docs.broadcom.com/docs/pub-005018


I did however find an older errata document at:


PEX 87xx Errata Version 1.14, September 25, 2015

https://docs.broadcom.com/doc/pub-005017


I will keep trying, and I will let you know if we manage to come across any

documents.


Thank you for your efforts.

Matthew

> Thanks,
> Alex
>