Re: [PATCH 04/17] gpu: nova-core: allocate PCI MSI vector during probe

[Danilo Krummrich]

On Fri Aug 14, 2026 at 12:29 AM CEST, John Hubbard wrote:
> On 8/13/26 3:26 PM, Danilo Krummrich wrote:
>> On Thu Aug 13, 2026 at 11:43 PM CEST, John Hubbard wrote:
>>> On 8/13/26 2:31 PM, Danilo Krummrich wrote:
>>>> On Sat Aug 8, 2026 at 5:11 AM CEST, John Hubbard wrote:
> ...
>>>> I was about to ask if we really need to bother with MSI and shouln't just go for
>>>> MSI-X only.
>>>
>>> I have an expensive, recent motherboard in my test machine, and it seems to only
>>> expose MSI for my Turing, Ampere and Blackwell GPUs, in the non-SRIOV configuration.
>>> So I'm thinking that MSI-X only would not work.
>> 
>> Interesting, I just checked one of my Ada and one of my Ampere GPUs and neither
>> reports MSI-X in the capabilities. Other devices show up with MSI-X though...
>> 
>> Are we sure it's not an endpoint limitation? I really thought it's not.
>
> I haven't really dug into it, because it seems very clear that regardless
> of the answer, we need to support both MSI and MSI-X in nova-core, right?

Apparently we do, and I'm a bit surprised by that. But it would still be
interesting to know what's the constraint, as this seems to be an endpoint
limitation. IOW, how can I exercise the MSI-X code?

>>>> But then saw that the commit message mentions broken VFIO setups; can you expand
>>>> on this a bit? Which setups is the commit message referring to?
>>>
>>> On Zhi Wang's Big Branch of Everything, he is able to run Windows guests in a
>>> vGPU VM on top of nova-core. That system requires MSI-X. And I broke it with
>>> an earlier internal version of this patchset.
>>>
>>> That information probably leaked into my commit message here. I can remove
>>> it. Because I don't think it is based on anything else.
>> 
>> So I guess we can also drop the INTx fallback? AFAIK SR-IOV VFs won't work with
>> INTx anyway.
>> 
> We could drop it. It's there mainly on General Principles, not because today's
> GPUs use it.