linux-kernel - Re: [PATCH 0/2] rust: pci: expose is_virtfn() and reject VFs in nova-core

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Message-ID: <e77bbcda-35a3-4ec6-ac24-316ab34a201a@nvidia.com>
Date: Tue, 30 Sep 2025 18:22:54 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alistair Popple <apopple@...dia.com>
Cc: Danilo Krummrich <dakr@...nel.org>,
 Alexandre Courbot <acourbot@...dia.com>,
 Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi <ttabi@...dia.com>,
 Zhi Wang <zhiw@...dia.com>, Surath Mitra <smitra@...dia.com>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 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 0/2] rust: pci: expose is_virtfn() and reject VFs in
 nova-core

On 9/30/25 5:29 PM, Alistair Popple wrote:
> On 2025-10-01 at 08:07 +1000, John Hubbard <jhubbard@...dia.com> 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.
>>
>> Add a method to check if a PCI device is a Virtual Function (VF). This
>> allows Rust drivers to determine whether a device is a VF created
>> through SR-IOV. This is required in order to implement VFIO, because
>> drivers such as NovaCore must only bind to Physical Functions (PFs) or
>> regular PCI devices. The VFs must be left unclaimed, so that a VFIO
>> kernel module can claim them.
> 
> Curiously based on a quick glance I didn't see any other drivers doing this
> which makes me wonder why we're different here. But it seems likely their
> virtual functions are supported by the same driver rather than requiring a
> different VF specific driver (or I glanced too quickly!).

I haven't checked into that, but it sounds reasonable.

> 
> I'm guessing the proposal is to fail the probe() function in nova-core for
> the VFs - I'm not sure but does the driver core continue to try probing other
> drivers if one fails probe()? It seems like this would be something best
> filtered on in the device id table, although I understand that's not possible
> today.

Yes, from my experience with building Nouveau and Nova and running both
on the same system, with 2 GPUs: when Nova gets probed first, because Nova
is a work in progress, however far it gets, it still fails the probe in
the end.

And then Nouveau gets probed, and claims the GPU.

thanks,
-- 
John Hubbard