linux-kernel - Re: [PATCH v4 01/10] PCI: Avoid saving error values for config space

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Message-ID: <aOc_k2MjZI6hYgKy@wunner.de>
Date: Thu, 9 Oct 2025 06:52:35 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Farhan Ali <alifm@...ux.ibm.com>
Cc: Benjamin Block <bblock@...ux.ibm.com>, linux-s390@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, alex.williamson@...hat.com,
	helgaas@...nel.org, clg@...hat.com, schnelle@...ux.ibm.com,
	mjrosato@...ux.ibm.com
Subject: Re: [PATCH v4 01/10] PCI: Avoid saving error values for config space

On Wed, Oct 08, 2025 at 02:55:56PM -0700, Farhan Ali wrote:
> > > On 10/8/2025 6:34 AM, Lukas Wunner wrote:
> > > > I also don't quite understand why the VM needs to perform a reset.
> > > > Why can't you just let the VM tell the host that a reset is needed
> > > > (PCI_ERS_RESULT_NEED_RESET) and then the host resets the device on
> > > > behalf of the VM?
> 
> The reset is not performed by the VM, reset is still done by the host. My
> approach for a VM to let the host know that reset was needed, was to
> intercept any reset instructions for the PCI device in QEMU. QEMU would then
> drive a reset via VFIO_DEVICE_RESET. Maybe I am missing something, but based
> on what we have today in vfio driver, we don't have a mechanism for
> userspace to reset a device other than VFIO_DEVICE_RESET and
> VFIO_PCI_DEVICE_HOT_RESET ioctls.

The ask is for the host to notify the VM of the ->error_detected() event
and the VM then responding with one of the "enum pci_ers_result" values.

If the VM returns PCI_ERS_RESULT_NEED_RESET from ->error_detected(),
the host knows that it shall reset the device.  There is no need
for the VM to initiate a reset through an ioctl.  Just integrate
with the existing error handling flow described in
Documentation/PCI/pci-error-recovery.rst and thereby use the
existing mechanism to reset the device.

Thanks,

Lukas