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

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Message-ID: <12076511-7113-4c53-83e8-92c5ea0eb125@nvidia.com>
Date: Tue, 30 Sep 2025 18:45:03 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alexandre Courbot <acourbot@...dia.com>,
 Danilo Krummrich <dakr@...nel.org>
Cc: 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>, 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>,
 Jason Gunthorpe <jgg@...dia.com>,
 Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH 0/2] rust: pci: expose is_virtfn() and reject VFs in
 nova-core

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