Nouveau - Oct 2025 - [PATCH v2 1/2] rust: pci: skip probing VFs if driver doesn't support VFs

On 10/2/25 10:40 AM, Danilo Krummrich wrote:
> On Thu Oct 2, 2025 at 7:37 PM CEST, Danilo Krummrich wrote:
>> On Thu Oct 2, 2025 at 7:05 PM CEST, Jason Gunthorpe wrote:
>>> On Thu, Oct 02, 2025 at 06:05:28PM +0200, Danilo Krummrich wrote:
>>>> On Thu Oct 2, 2025 at 5:23 PM CEST, Jason Gunthorpe wrote:
>>>>> This is not what I've been told, the VF driver has
significant
>>>>> programming model differences in the NVIDIA model, and
supports
>>>>> different commands.
>>>>
>>>> Ok, that means there are some more fundamental differences
between the host PF
>>>> and the "VM PF" code that we have to deal with.
>>>
>>> That was my understanding.
>>>  
>>>> But that doesn't necessarily require that the VF parts of
the host have to be in
>>>> nova-core as well, i.e. with the information we have we can
differentiate
>>>> between PF, VF and PF in the VM (indicated by a device
register).
>>>
>>> I'm not entirely sure what you mean by this..
>>>
>>> The driver to operate the function in "vGPU" mode as
indicated by the
>>> register has to be in nova-core, since there is only one device ID.
>>
>> Yes, the PF driver on the host and the PF (from VM perspective) driver
in the VM
>> have to be that same. But the VF driver on the host can still be a
seaparate
>> one.
>>
>>>>> If you look at the VFIO driver RFC it basically does no
mediation, it
>>>>> isn't intercepting MMIO - the guest sees the BARs
directly. Most of
>>>>> the code is "profiling" from what I can tell.
Some config space
>>>>> meddling.
>>>>
>>>> Sure, there is no mediation in that sense, but it needs quite
some setup
>>>> regardless, no?
>>>>
>>>> I thought there is a significant amount of semantics that is
different between
>>>> booting the PF and the VF on the host.
>>>
>>> I think it would be good to have Zhi clarify more of this, but from
>>> what I understand are at least three activites comingled all
together:
>>>
>>>  1) Boot the PF in "vGPU" mode so it can enable SRIOV
>>
For this, we could pass a kernel module parameter to nova-core.
>> Ok, this might be where the confusion above comes from. When I talk
about
>> nova-core in vGPU mode I mean nova-core running in the VM on the (from
VM
>> perspective) PF.
>>
>> But you seem to mean nova-core running on the host PF with vGPU on top?
That of
>> course has to be in nova-core.
>>
>>>  2) Enable SRIOV and profile VFs to allocate HW resources to them
>>
>> I think that's partially in nova-core and partially in vGPU;
nova-core providing
>> the abstraction of the corresponding firmware / hardware interfaces and
vGPU
>> controlling the semantics of the resource handling?
>>
>> This is what I thought vGPU has a secondary part for where it binds to
nova-core
>> through the auxiliary bus, i.e. vGPU consisting out of two drivers
actually; the
>> VFIO parts and a "per VF resource controller".
> 
> Forgot to add: But I think Zhi explained that this is not necessary and can
be
> controlled by the VFIO driver, i.e. the PCI driver that binds to the VF
itself.
Yes, this is the direction that I originally (3 whole days ago, haha) had in
mind,
after talking with Zhi and a few others: nova-core handles PFs, and the VFIO
driver
handles the VFs, and use the "is virtual" logic to sort them out.

Looking forward to Zhi's reaction to the other approach that you and Jason
have been debating. This is all very educational to me, as a VFIO newbie. :)
> 
>>>  3) VFIO variant driver to convert the VF into a "VM PF"
with whatever
>>>     mediation and enhancement needed
>>
>> That should be vGPU only land.
> 
thanks,
-- 
John Hubbard

On Thu, Oct 02, 2025 at 10:49:21AM -0700, John Hubbard
wrote:
> > Forgot to add: But I think Zhi explained that this is not necessary
and can be
> > controlled by the VFIO driver, i.e. the PCI driver that binds to the
VF itself.
> 
> Yes, this is the direction that I originally (3 whole days ago, haha) had
in mind,
> after talking with Zhi and a few others: nova-core handles PFs, and the
VFIO driver
> handles the VFs, and use the "is virtual" logic to sort them out.
To be clear, no matter what the VFIO driver bound to the VF should not
become entangled with any aux devices.

The VFIO VF driver uses pci_iov_get_pf_drvdata() to reach into the PF
to request the PF's help. Eg for live migration or things of that
nature.

My point here is that generally we don't put profiling code in the
VFIO driver and then use pci_iov_get_pf_drvdata() to access the PF do
actually do the profiling.

The VF cannot/should not control profiling of itself - that would be a
security problem once it is assigned to a VM.

So the profiling resides entirely inside the PF world and should
operate without VFIO. As I've said this design is compatible with VFs
for containers and so on. So it is the strongly preferred design
pattern.

Jason