linux-kernel - Re: [PATCH] pci: account for sysfs-disabled reset in pci_{slot,bus}

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Message-ID: <20250113204200.GZ5556@nvidia.com>
Date: Mon, 13 Jan 2025 16:42:00 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nishanth Aravamudan <naravamudan@...dia.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Raphael Norwitz <raphael.norwitz@...anix.com>,
	Amey Narkhede <ameynarkhede03@...il.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>, kvm@...r.kernel.org
Subject: Re: [PATCH] pci: account for sysfs-disabled reset in
 pci_{slot,bus}_resettable

On Mon, Jan 06, 2025 at 03:52:31PM -0600, Nishanth Aravamudan wrote:
> vfio_pci_ioctl_get_pci_hot_reset_info checks if either the vdev's slot
> or bus is not resettable by calling pci_probe_reset_{slot,bus}. Those
> functions in turn call pci_{slot,bus}_resettable() to see if the PCI
> device supports reset.

This change makes sense to me, but..

> However, commit d88f521da3ef ("PCI: Allow userspace to query and set
> device reset mechanism") added support for userspace to disable reset of
> specific PCI devices (by echo'ing "" into reset_method) and
> pci_{slot,bus}_resettable methods do not check pci_reset_supported() to
> see if userspace has disabled reset. Therefore, if an administrator
> disables PCI reset of a specific device, but then uses vfio-pci with
> that device (e.g. with qemu), vfio-pci will happily end up issuing a
> reset to that device.

How does vfio-pci endup issuing a reset? It looked like all the paths
are blocked in the pci core with pci_reset_supported()? Is there also
a path that vfio is calling that is missing a pci_reset_supported()
check? If yes that should probably be fixed in another patch.

Or do you mean that VFIO tries to do a reset but it fails and nothing
happens, the real issue is that hot_reset_info is reporting incorrect
information to userspace?

Jason