Re: [PATCH] rust: pci: rework device enabling API

[Maurice Hieronymus <mhi@mailbox.org>]

On Sat Jul 11, 2026 at 5:25 PM CEST, Maurice Hieronymus wrote:
> I went ahead and sent a series implementing the second option, a
> public flags bitmap with generated accessors following the driver
> core precedent, converting is_busmaster and broken_parity_status:
>
> https://lore.kernel.org/r/20260711-pci-dev-flags-v1-0-2fcf2811138c@mailbox.org

A status update on that, and two questions.

v2 [1] moved the bit into `priv_flags` instead, as Lukas preferred. He
then asked me to convert xen-pciback's two direct `dev->is_busmaster = 0`
assignments to `pci_clear_master()`, so that nothing outside drivers/pci
writes the flag [2].

That conversion is not behavior-preserving: the assignments clear the
software flag only, while `pci_clear_master()` also clears
PCI_COMMAND_MASTER in config space. Commit 7681f31ec9cd ("xen/pciback:
Don't disable PCI_COMMAND on PCI device reset.") deliberately removed a
PCI_COMMAND write from directly above the first assignment, so putting
one back at that spot looked wrong to me.

I said so in that thread on Jul 18 [3], and asked the xen maintainers
directly in a separate mail on Jul 23 [4]. Neither has had a reply, so
the C-side prerequisite has not moved since.

On proceeding on the Rust side in the meantime:

I would like to keep the enable/disable rework in the EDU series rather
than drop it. `enable_device_mem()` has no disable counterpart, so the
enable count is never dropped on unbind, and the sample does not come
back up after rmmod + modprobe.

The races the guard would expose are not introduced by this series.
`is_busmaster` shares one bitfield word in `struct pci_dev` with a
number of other flags, and those are written from unrelated contexts, so
several of them can be updated concurrently with it.
`broken_parity_status` is only the clearest example:
`broken_parity_status_store()` (drivers/pci/pci-sysfs.c) writes it from
sysfs without taking any lock at all, so userspace can already race an
`is_busmaster` update today.

So my first question: would you take the `DeviceEnableGuard` design
while that word is still a C bitfield, given that it can already be
raced from userspace today, and let the EDU sample land on top of it,
replacing `rust_driver_pci` as you asked? Or do you want the bitops
conversion merged first, in which case the series stays blocked on
linux-pci.

My second question is a behavioural one, since I do not have much
experience here: what is the accepted way forward when a list does not
respond? I do not want to resend the same question every week and turn
into noise, but I also do not want to guess at xen-pciback semantics and
break someone's setup.

[1] https://lore.kernel.org/linux-pci/20260714-pci-dev-flags-v2-1-a1d7dc441cf3@mailbox.org/
[2] https://lore.kernel.org/linux-pci/alcSjoypegNolW48@wunner.de/
[3] https://lore.kernel.org/linux-pci/DK1UXXRGW737.2IVTYCHPRM9ET@mailbox.org/
[4] https://lore.kernel.org/linux-pci/20260723161638.9838-1-mhi@mailbox.org/

Best,

Maurice