Re: [PATCH v20 4/4] PCI/MSI: Enable memory decoding before restoring MSI-X messages

[Farhan Ali <alifm@linux.ibm.com>]

On 6/23/2026 4:28 AM, Niklas Schnelle wrote:
> On Mon, 2026-06-22 at 13:49 -0700, Farhan Ali wrote:
>> On 6/22/2026 1:29 PM, Thomas Gleixner wrote:
>>> On Mon, Jun 22 2026 at 10:18, Farhan Ali wrote:
>>>> The current MSI-X restoration path assumes the Command register Memory bit
>>>> is enabled when writing MSI-X messages. But its possible the last saved and
>>>> restored state of device may not have the Memory bit enabled, even if a
>>>> device driver later enables Memory bit and MSI-X. Attempting to access
>>>> Memory space without Memory bit enabled can lead to Unsupported Request
>>>> (UR) from the device. Fix this by enabling Memory bit and restore
>>>> it afterwards.
>>>>
>>>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>>>> ---
>>>>
> --- 8< ---
>>>> @@ -882,6 +883,8 @@ void __pci_restore_msix_state(struct pci_dev *dev)
>>>>    	pci_intx_for_msi(dev, 0);
>>>>    	pci_msix_clear_and_set_ctrl(dev, 0,
>>>>    				PCI_MSIX_FLAGS_ENABLE | PCI_MSIX_FLAGS_MASKALL);
>>>> +	pci_read_config_word(dev, PCI_COMMAND, &cmd);
>>>> +	pci_write_config_word(dev, PCI_COMMAND, cmd | PCI_COMMAND_MEMORY);
>>> Can we please have a comment there which explains this? Three month down
>>> the road this will results in head scratching otherwise.
>>>
>>> I agree with Niklas that this wants a Fixes and a Cc:stable tag.
>>>
>>> Other than that:
>>>
>>> Reviewed-by: Thomas Gleixner <tglx@kernel.org>
>>>
>>> Thanks,
>>>
>>>           tglx
>>>
>> I can add a comment, how about something like below?
>>
>> "The restored device state may not have Memory decoding enabled in
>> Command register. Since the MSI-X was enabled for the device, enable
>> Memory decoding before restoring MSI-X"
> Missing "the" before "Command register" other than that this sounds
> good to me and I agree with Thomas that a comment makes sense here.
>
>> For the Fixes tag, do you have a suggestion for a commit? This behavior
>> has been present since 41017f0cac92 ("[PATCH] PCI: MSI(X) save/restore
>> for suspend/resume" which introduced these restore functions. So should
>> be Fixes against 41017f0cac92?
> I'm not sure but my guess would be that for suspend/resume this isn't
> an issue since the suspend part would save the state with Memory Space
> enabled. So it wouldn't be broken in this original use-case. I think
> the problem only occurs in case restore is done for error recovery.
> Might it make sense to have the Fixes tag point to a2f1e22390ac
> ("PCI/ERR: Ensure error recoverability at all times") even though that
> only helps to expose the issue?
>
> Thanks,
> Niklas

Yeah, commit a2f1e22390ac ("PCI/ERR: Ensure error recoverability at all 
times") exposes this issue. I am okay if we want Fixes tag to point to 
this change, unless someone objects. I will also add a note in the 
commit message to specify a2f1e22390ac exposes this issue.

Thanks

Farhan