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

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Message-ID: <422d6b1d-d2be-4f78-a973-05a4316e62f3@linux.ibm.com>
Date: Fri, 12 Sep 2025 10:19:41 -0700
From: Farhan Ali <alifm@...ux.ibm.com>
To: Benjamin Block <bblock@...ux.ibm.com>
Cc: 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, schnelle@...ux.ibm.com,
        mjrosato@...ux.ibm.com
Subject: Re: [PATCH v3 03/10] PCI: Allow per function PCI slots


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