Re: [PATCH v11 1/9] PCI: Allow per function PCI slots

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Mar 16, 2026 at 12:15:36PM -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.

I think this "pass-through" is from the hypervisor to Linux, i.e.,
what we think of as the host kernel, right?

> 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().

This alludes to the patch fixing a reset issue, but I think it should
be more prominent, e.g., the reset and leak fixes could be a separate
paragraph.  The subject line should also mention at least the reset
fix.

> 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. Since we can use all 8 bits
> for slot 'number' (for ARI devices), change slot 'number' u16 to
> account for special values -1 and PCI_SLOT_ALL_DEVICES.
> ...

>  static ssize_t address_read_file(struct pci_slot *slot, char *buf)
>  {
> -	if (slot->number == 0xff)
> +	if (slot->number == (u16)-1)

This "-1" is mentioned in the commit log, but I don't know where it
came from.  I guess we must assign -1 as a default somewhere?  Could
this be a #define to connect that assignment with this test?

>  		return sysfs_emit(buf, "%04x:%02x\n",
>  				  pci_domain_nr(slot->bus),
>  				  slot->bus->number);