Re: [PATCH v3 03/10] PCI: Allow per function PCI slots

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

On 9/12/2025 5:23 AM, Benjamin Block wrote:
> On Thu, Sep 11, 2025 at 11:33:00AM -0700, Farhan Ali wrote:
>> On s390 systems, which use a machine level hypervisor, PCI devices are
>> always accessed through a form of PCI pass-through which fundamentally
>> operates on a per PCI function granularity. This is also reflected in the
>> s390 PCI hotplug driver which creates hotplug slots for individual PCI
>> functions. Its reset_slot() function, which is a wrapper for
>> zpci_hot_reset_device(), thus also resets individual functions.
>>
>> Currently, the kernel's PCI_SLOT() macro assigns the same pci_slot object
>> to multifunction devices. This approach worked fine on s390 systems that
>> only exposed virtual functions as individual PCI domains to the operating
>> system.  Since commit 44510d6fa0c0 ("s390/pci: Handling multifunctions")
>> s390 supports exposing the topology of multifunction PCI devices by
>> grouping them in a shared PCI domain. When attempting to reset a function
>> through the hotplug driver, the shared slot assignment causes the wrong
>> function to be reset instead of the intended one. It also leaks memory as
>> we do create a pci_slot object for the function, but don't correctly free
>> it in pci_slot_release().
>>
>> Add a flag for struct pci_slot to allow per function PCI slots for
>> functions managed through a hypervisor, which exposes individual PCI
>> functions while retaining the topology.
>>
>> Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
> Stable tag?
> Reseting the wrong PCI function sounds bad enough.

That's a fair point. This is definitely broken for NETD devices 
(https://www.ibm.com/docs/en/linux-on-systems?topic=express-direct-mode). 
Will cc stable.

Thanks
Farhan