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

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.
> > 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
 2) Enable SRIOV and profile VFs to allocate HW resources to them
 3) VFIO variant driver to convert the VF into a "VM PF" with whatever
    mediation and enhancement needed
>From a broad perspective we in the kernel have put #2 outside VFIO
because all of that is actually run through the PF and doesn't use the
VF at all.

#3 is the vfio driver and I would like it if vfio drivers restrained
themselves to focus on the mediation, live migration and things like
that which are directly related to VFIO..
> Also, the idea was to use a layered approach, i.e. let nova-core
> serve as an abstraction layer, where the DRM and VFIO parts can be
> layered on top of.
Yes, I think everyone is good with some version of this..

A big question in my mind is where do you put #2, and what uapi does
it provide. It has to layer on top of nova-core because it has to use
the PF to do profiling.

I'm not a fan of the vfio based sysfs as uAPI for #2, for reasons
touched on in this thread.

NIC drivers are using fwctl and devlink for profiling, managed by the
PF driver. I think I'd want to here reasons why those interfaces
cannot be used here.

Jason

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
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".
>  3) VFIO variant driver to convert the VF into a "VM PF" with
whatever
>     mediation and enhancement needed
That should be vGPU only land.