Nouveau - Oct 2025 - [PATCH 0/2] rust: pci: expose is_virtfn() and reject VFs in nova-core

On Wed Oct 1, 2025 at 10:26 AM JST, John Hubbard wrote:
> On 9/30/25 5:26 PM, Alexandre Courbot wrote:
>> On Wed Oct 1, 2025 at 7:07 AM JST, John Hubbard wrote:
>>> Post-Kangrejos, the approach for NovaCore + VFIO has changed a bit:
the
>>> idea now is that VFIO drivers, for NVIDIA GPUs that are supported
by
>>> NovaCore, should bind directly to the GPU's VFs. (An earlier
idea was to
>>> let NovaCore bind to the VFs, and then have NovaCore call into the
upper
>>> (VFIO) module via Aux Bus, but this turns out to be awkward and is
no
>>> longer in favor.) So, in order to support that:
>>>
>>> Nova-core must only bind to Physical Functions (PFs) and regular
PCI
>>> devices, not to Virtual Functions (VFs) created through SR-IOV.
>> 
>> Naive question: will guests also see the passed-through VF as a VF? If
>> so, wouldn't this change also prevents guests from using Nova?
>
> I'm also new to this area. I would expect that guests *must* see
> these as PFs, otherwise...nothing makes any sense.
But if the guest sees the passed-through VF as a PF, won't it try to
do things it is not supposed to do like loading the GSP firmware (which
is managed by the host)?

On 9/30/25 6:39 PM, Alexandre Courbot wrote:
> On Wed Oct 1, 2025 at 10:26 AM JST, John Hubbard wrote:
>> On 9/30/25 5:26 PM, Alexandre Courbot wrote:
>>> On Wed Oct 1, 2025 at 7:07 AM JST, John Hubbard wrote:
>>>> Post-Kangrejos, the approach for NovaCore + VFIO has changed a
bit: the
>>>> idea now is that VFIO drivers, for NVIDIA GPUs that are
supported by
>>>> NovaCore, should bind directly to the GPU's VFs. (An
earlier idea was to
>>>> let NovaCore bind to the VFs, and then have NovaCore call into
the upper
>>>> (VFIO) module via Aux Bus, but this turns out to be awkward and
is no
>>>> longer in favor.) So, in order to support that:
>>>>
>>>> Nova-core must only bind to Physical Functions (PFs) and
regular PCI
>>>> devices, not to Virtual Functions (VFs) created through SR-IOV.
>>>
>>> Naive question: will guests also see the passed-through VF as a VF?
If
>>> so, wouldn't this change also prevents guests from using Nova?
>>
>> I'm also new to this area. I would expect that guests *must* see
>> these as PFs, otherwise...nothing makes any sense.
> 
> But if the guest sees the passed-through VF as a PF, won't it try to
> do things it is not supposed to do like loading the GSP firmware (which
> is managed by the host)?
Yes. A non-paravirtualized guest will attempt to behave just like a
bare metal driver would behave. It's the job of the various layers
of virtualization to intercept and modify such things appropriately.

Looking ahead: if the VFIO experts come back and tell us that guests
see these as VFs, then there is still a way forward, because we
talked about loading nova-core with a "vfio_mode" kernel module
parameter. So then it becomes "if vfio_mode, then skip VFs".


thanks,
-- 
John Hubbard