linux-kernel - Re: [PATCH v2 1/2] rust: pci: skip probing VFs if driver doesn't support VFs

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Message-Id: <DD80R10HBCHR.1BZNEAAQI36LE@kernel.org>
Date: Thu, 02 Oct 2025 19:40:07 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Jason Gunthorpe" <jgg@...dia.com>
Cc: "John Hubbard" <jhubbard@...dia.com>, "Alexandre Courbot"
 <acourbot@...dia.com>, "Joel Fernandes" <joelagnelf@...dia.com>, "Timur
 Tabi" <ttabi@...dia.com>, "Alistair Popple" <apopple@...dia.com>, "Zhi
 Wang" <zhiw@...dia.com>, "Surath Mitra" <smitra@...dia.com>, "David Airlie"
 <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>, "Alex Williamson"
 <alex.williamson@...hat.com>, "Bjorn Helgaas" <bhelgaas@...gle.com>,
 Krzysztof Wilczyński <kwilczynski@...nel.org>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun
 Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 <nouveau@...ts.freedesktop.org>, <linux-pci@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, "LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] rust: pci: skip probing VFs if driver doesn't
 support VFs

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
>
> 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.

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