Re: [PATCH v1 1/3] PCI: Allow per function PCI slots

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

On 10/21/2025 5:49 AM, Niklas Schnelle wrote:
> On Mon, 2025-10-20 at 12:01 -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")
>> Cc: stable@vger.kernel.org
>> Suggested-by: Niklas Schnelle <schnelle@linux.ibm.com>
>> Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>> ---
>>   drivers/pci/hotplug/s390_pci_hpc.c | 10 ++++++++--
>>   drivers/pci/pci.c                  |  5 +++--
>>   drivers/pci/slot.c                 | 14 +++++++++++---
>>   include/linux/pci.h                |  1 +
>>   4 files changed, 23 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/pci/hotplug/s390_pci_hpc.c b/drivers/pci/hotplug/s390_pci_hpc.c
>> index d9996516f49e..8b547de464bf 100644
>> --- a/drivers/pci/hotplug/s390_pci_hpc.c
>> +++ b/drivers/pci/hotplug/s390_pci_hpc.c
>> @@ -126,14 +126,20 @@ static const struct hotplug_slot_ops s390_hotplug_slot_ops = {
>>   
>>   int zpci_init_slot(struct zpci_dev *zdev)
>>   {
>> +	int ret;
>>   	char name[SLOT_NAME_SIZE];
>>   	struct zpci_bus *zbus = zdev->zbus;
>>   
>>   	zdev->hotplug_slot.ops = &s390_hotplug_slot_ops;
>>   
>>   	snprintf(name, SLOT_NAME_SIZE, "%08x", zdev->fid);
>> -	return pci_hp_register(&zdev->hotplug_slot, zbus->bus,
>> -			       zdev->devfn, name);
>> +	ret = pci_hp_register(&zdev->hotplug_slot, zbus->bus,
>> +				zdev->devfn, name);
>> +	if (ret)
>> +		return ret;
>> +
>> +	zdev->hotplug_slot.pci_slot->per_func_slot = 1;
> I think the way this works is a bit odd. Due to the order of setting
> the flag pci_create_slot() in pci_hp_register() tries to match using
> the wrong per_func_slot == 0. This doesn't really cause mismatches
> though because the slot->number won't match the PCI_SLOT(dev->devfn)
> except for the slot->number 0 where it is fine.
>
> One way to improve(?) on this is to have a per_func_slot flag also in
> the struct hotplug_slot and then copy it over into the newly created
> struct pci_slot. But then we have this flag twice. Or maybe this really
> should be an argument to pci_create_slot()?

This would still work as we associate the struct pci_dev to struct 
pci_slot in pci_dev_assign_slot(), when we would have the flag set. But 
I do see your point that there is room for improvement here. As 
discussed offline we can maybe have the flag in struct pci_bus since we 
already have the slots list. This would allow us to set the flag for 
zpci devices at the creation of the pci_bus. And can be used by 
pci_create_slot() and pci_dev_assign_slot() to correctly set the slot 
for the pci dev. Will post a v2 with this.

Thanks

Farhan