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

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Message-ID: <20251002234010.GI3299207@nvidia.com>
Date: Thu, 2 Oct 2025 20:40:10 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
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 02, 2025 at 11:32:44PM +0200, Danilo Krummrich wrote:

> So, when we call pdev.physfn().drvdata_borrow::<NovaCore>() the checks are
> included already.

I'm not keen on hiding this reasoning inside an physfn() accessor like
this. ie one that returns a Device<Bound>. The reasoning for this is
tricky and special. We have enough cases where physfn won't be a bound
driver. I think it is big stretch just to declare that unconditionally
safe.

There is a reason pci_iov_get_pf_drvdata() has such a big comment..

So I'd rather see you follow the C design and have an explicit helper
function to convert a VF bound device to a PF bound device and check
the owner, basically split up pci_iov_get_pf_drvdata() into a part to
get the struct device and an inline to get the drvdata. Rust still has an
ops pointer it can pass in so it can be consistent with the C code
even if it does another check inside its drvdata_borrow.

This way we keep the reasoning and explanation in one place.

Jason