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

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Message-ID: <6c514ba0-7910-4770-903f-62c3e827a40b@linux.ibm.com>
Date: Wed, 8 Oct 2025 14:55:56 -0700
From: Farhan Ali <alifm@...ux.ibm.com>
To: Lukas Wunner <lukas@...ner.de>
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 10/8/2025 11:14 AM, Lukas Wunner wrote:
> 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?
>
>
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.