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

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Message-ID: <aOaqEhLOzWzswx8O@wunner.de>
Date: Wed, 8 Oct 2025 20:14:42 +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 10:56:35AM -0700, Farhan Ali wrote:
> On 10/8/2025 6:34 AM, Lukas Wunner wrote:
> > I'm not sure yet.  Let's back up a little:  I'm missing an
> > architectural description how you're intending to do error
> > recovery in the VM.  If I understand correctly, you're
> > informing the VM of the error via the ->error_detected() callback.
> > 
> > You're saying you need to check for accessibility of the device
> > prior to resetting it from the VM, does that mean you're attempting
> > a reset from the ->error_detected() callback?
> > 
> > According to Documentation/PCI/pci-error-recovery.rst, the device
> > isn't supposed to be considered accessible in ->error_detected().
> > The first callback which allows access is ->mmio_enabled().
> > 
> 
> The ->error_detected() callback is used to inform userspace of an error. In
> the case of a VM, using QEMU as a userspace, once notified of an error QEMU
> will inject an error into the guest in s390x architecture specific way [1]
> (probably should have linked the QEMU series in the cover letter). Once
> notified of the error VM's device driver will drive the recovery action. The
> recovery action require a reset of the device and on s390x PCI devices are
> reset using architecture specific instructions (zpci_device_hot_reset()).

According to Documentation/PCI/pci-error-recovery.rst:

   "STEP 1: Notification
    --------------------
    Platform calls the error_detected() callback on every instance of
    every driver affected by the error.
    At this point, the device might not be accessible anymore, [...]
    it gives the driver a chance to cleanup, waiting for pending stuff
    (timers, whatever, etc...) to complete; it can take semaphores,
    schedule, etc... everything but touch the device."
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

And yet you're touching the device by trying to reset it.

The code you're introducing in patch [01/10] only becomes necessary
because you're not following the above-quoted protocol.  If you
follow the protocol, patch [01/10] becomes unnecessary.

> > 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?

Could you answer this question please?

Thanks,

Lukas