linux-kernel - Re: [REGRESSION] Too-low frequency limit for AMD GPU PCI-passed-through to Windows VM

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Message-ID: <bc714e87-d1dc-cdda-5a29-25820faaff40@leemhuis.info>
Date:   Fri, 18 Mar 2022 08:01:31 +0100
From:   Thorsten Leemhuis <regressions@...mhuis.info>
To:     Paul Menzel <pmenzel@...gen.mpg.de>,
        James Turner <linuxkernel.foss@...rc-none.turner.link>
Cc:     Xinhui Pan <Xinhui.Pan@....com>, regressions@...ts.linux.dev,
        kvm@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
        Lijo Lazar <lijo.lazar@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org,
        Alexander Deucher <Alexander.Deucher@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Alex Deucher <alexdeucher@...il.com>,
        Christian König <Christian.Koenig@....com>
Subject: Re: [REGRESSION] Too-low frequency limit for AMD GPU
 PCI-passed-through to Windows VM

On 18.03.22 06:43, Paul Menzel wrote:
>
> Am 17.03.22 um 13:54 schrieb Thorsten Leemhuis:
>> On 13.03.22 19:33, James Turner wrote:
>>>
>>>> My understanding at this point is that the root problem is probably
>>>> not in the Linux kernel but rather something else (e.g. the machine
>>>> firmware or AMD Windows driver) and that the change in f9b7f3703ff9
>>>> ("drm/amdgpu/acpi: make ATPX/ATCS structures global (v2)") simply
>>>> exposed the underlying problem.
>>
>> FWIW: that in the end is irrelevant when it comes to the Linux kernel's
>> 'no regressions' rule. For details see:
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/admin-guide/reporting-regressions.rst
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/process/handling-regressions.rst
>>
>>
>> That being said: sometimes for the greater good it's better to not
>> insist on that. And I guess that might be the case here.
> 
> But who decides that?

In the end afaics: Linus. But he can't watch each and every discussion,
so it partly falls down to people discussing a regression, as they can
always decide to get him involved in case they are unhappy with how a
regression is handled. That obviously includes me in this case. I simply
use my best judgement in such situations. I'm still undecided if that
path is appropriate here, that's why I wrote above to see what James
would say, as he afaics was the only one that reported this regression.

> Running stuff in a virtual machine is not that uncommon.

No, it's about passing through a GPU to a VM, which is a lot less common
-- and afaics an area where blacklisting GPUs on the host to pass them
through is not uncommon (a quick internet search confirmed that, but I
might be wrong there).

> Should the commit be reverted, and re-added with a more elaborate commit
> message documenting the downsides?
> 
> Could the user be notified somehow? Can PCI passthrough and a loaded
> amdgpu driver be detected, so Linux warns about this?
>
> Also, should this be documented in the code?
>
>>> I'm not sure where to go from here. This issue isn't much of a concern
>>> for me anymore, since blacklisting `amdgpu` works for my machine. At
>>> this point, my understanding is that the root problem needs to be fixed
>>> in AMD's Windows GPU driver or Dell's firmware, not the Linux kernel. If
>>> any of the AMD developers on this thread would like to forward it to the
>>> AMD Windows driver team, I'd be happy to work with AMD to fix the issue
>>> properly.
> 
> (Thorsten, your mailer mangled the quote somehow 

Kinda, but it IIRC was more me doing something stupid with my mailer.
Sorry about that.

> – I reformatted it –,

thx!

> which is too bad, as this message is shown when clicking on the link
> *marked invalid* in the regzbot Web page [1]. (The link is a very nice
> feature.)
> 
>> In that case I'll drop it from the list of regressions, unless what I
>> wrote above makes you change your mind.
>>
>> #regzbot invalid: firmware issue exposed by kernel change, user seems to
>> be happy with a workaround
>>
>> Thx everyone who participated in handling this.
> 
> Should the regression issue be re-opened until the questions above are
> answered, and a more user friendly solution is found?

I'll for now will just continue to watch this discussion and see what
happens.

> [1]: https://linux-regtracking.leemhuis.info/regzbot/resolved/

Ciao, Thorsten