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

[Danilo Krummrich]

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:
>>> Allocate a single PCI MSI interrupt vector in the probe path.
>>>
>>> Try MSI/MSI-X first. If that fails (possible in broken VFIO setups),
>>> fall back to INTx with a dev_warn so the issue is visible in dmesg.
>>> The allocation is devres-managed and automatically freed on unbind.
>> 
>> [...]
>> 
>>> +pub(crate) fn alloc_vector(pdev: &pci::Device<Bound>) -> Result<pci::IrqVector<'_>> {
>>> +    let msi_types = IrqTypes::default().with(IrqType::Msi).with(IrqType::MsiX);
>> 
>> 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.

>> 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.

>>> +
>>> +    let irq_vectors = match pdev.alloc_irq_vectors(1, 1, msi_types) {
>>> +        Ok(vecs) => vecs,
>>> +        Err(_) => {
>>> +            dev_warn!(pdev.as_ref(), "MSI not available, falling back to INTx\n");
>>> +            pdev.alloc_irq_vectors(1, 1, IrqTypes::default().with(IrqType::Intx))?
>>> +        }
>>> +    };
>>> +
>>> +    irq_vectors.vector(0)
>>> +}