Re: [PATCH v14 1/7] PCI: Allow per function PCI slots to fix slot reset on s390

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

On 5/4/2026 8:52 AM, Gerd Bayer wrote:
> On Tue, 2026-04-21 at 09:30 -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. This creates a problem when resetting
>> a function through the hotplug driver's slot_reset() interface.
>>
>> 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().
>>
> Hi Farhan,
>
> sorry for jumping this late into reviewing this, but I think I'd prefer
> a different approach than extending the slot member to u16 to make the
> full range of 256 usable:
>
>> 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.
>>
>> Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
>> Cc: stable@vger.kernel.org
>> Suggested-by: Niklas Schnelle <schnelle@linux.ibm.com>
>> Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>> ---
>>   drivers/pci/hotplug/rpaphp_slot.c |  2 +-
>>   drivers/pci/pci.c                 |  5 +++--
>>   drivers/pci/slot.c                | 33 +++++++++++++++++++++++--------
>>   include/linux/pci.h               |  8 ++++++--
>>   4 files changed, 35 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/pci/hotplug/rpaphp_slot.c b/drivers/pci/hotplug/rpaphp_slot.c
>> index 67362e5b9971..92eabf5f61b9 100644
>> --- a/drivers/pci/hotplug/rpaphp_slot.c
>> +++ b/drivers/pci/hotplug/rpaphp_slot.c
>> @@ -84,7 +84,7 @@ int rpaphp_register_slot(struct slot *slot)
>>   	struct hotplug_slot *php_slot = &slot->hotplug_slot;
>>   	u32 my_index;
>>   	int retval;
>> -	int slotno = -1;
>> +	int slotno = PCI_SLOT_PLACEHOLDER;
>>   
>>   	dbg("%s registering slot:path[%pOF] index[%x], name[%s] pdomain[%x] type[%d]\n",
>>   		__func__, slot->dn, slot->index, slot->name,
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index 8f7cfcc00090..d0c9f0166af5 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -4865,8 +4865,9 @@ static int pci_reset_hotplug_slot(struct hotplug_slot *hotplug, bool probe)
>>   
>>   static int pci_dev_reset_slot_function(struct pci_dev *dev, bool probe)
>>   {
>> -	if (dev->multifunction || dev->subordinate || !dev->slot ||
>> -	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET)
>> +	if (dev->subordinate || !dev->slot ||
>> +	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET ||
>> +	    (dev->multifunction && !dev->slot->per_func_slot))
>>   		return -ENOTTY;
>>   
>>   	return pci_reset_hotplug_slot(dev->slot->hotplug, probe);
>> diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
>> index e0b7fb43423c..3f6e5dce27a0 100644
>> --- a/drivers/pci/slot.c
>> +++ b/drivers/pci/slot.c
>> @@ -37,7 +37,7 @@ static const struct sysfs_ops pci_slot_sysfs_ops = {
>>   
>>   static ssize_t address_read_file(struct pci_slot *slot, char *buf)
>>   {
>> -	if (slot->number == 0xff)
>> +	if (slot->number == (u16)PCI_SLOT_PLACEHOLDER)
>>   		return sysfs_emit(buf, "%04x:%02x\n",
>>   				  pci_domain_nr(slot->bus),
>>   				  slot->bus->number);
>> @@ -72,6 +72,23 @@ static ssize_t cur_speed_read_file(struct pci_slot *slot, char *buf)
>>   	return bus_speed_read(slot->bus->cur_bus_speed, buf);
>>   }
>>   
>> +static bool pci_dev_matches_slot(struct pci_dev *dev, struct pci_slot *slot)
>> +{
>> +	if (slot->per_func_slot)
>> +		return dev->devfn == slot->number;
>> +
>> +	return slot->number == PCI_SLOT_ALL_DEVICES ||
>> +		PCI_SLOT(dev->devfn) == slot->number;
>> +}
>> +
>> +static bool pci_slot_enabled_per_func(void)
>> +{
>> +	if (IS_ENABLED(CONFIG_S390))
>> +		return true;
>> +
>> +	return false;
>> +}
>> +
>>   static void pci_slot_release(struct kobject *kobj)
>>   {
>>   	struct pci_dev *dev;
>> @@ -82,8 +99,7 @@ static void pci_slot_release(struct kobject *kobj)
>>   
>>   	down_read(&pci_bus_sem);
>>   	list_for_each_entry(dev, &slot->bus->devices, bus_list)
>> -		if (slot->number == PCI_SLOT_ALL_DEVICES ||
>> -		    PCI_SLOT(dev->devfn) == slot->number)
>> +		if (pci_dev_matches_slot(dev, slot))
>>   			dev->slot = NULL;
>>   	up_read(&pci_bus_sem);
>>   
>> @@ -176,8 +192,7 @@ void pci_dev_assign_slot(struct pci_dev *dev)
>>   
>>   	mutex_lock(&pci_slot_mutex);
>>   	list_for_each_entry(slot, &dev->bus->slots, list)
>> -		if (slot->number == PCI_SLOT_ALL_DEVICES ||
>> -		    PCI_SLOT(dev->devfn) == slot->number)
>> +		if (pci_dev_matches_slot(dev, slot))
>>   			dev->slot = slot;
>>   	mutex_unlock(&pci_slot_mutex);
>>   }
>> @@ -256,7 +271,7 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>>   
>>   	mutex_lock(&pci_slot_mutex);
>>   
>> -	if (slot_nr == -1)
>> +	if (slot_nr == PCI_SLOT_PLACEHOLDER)
>>   		goto placeholder;
>>   
>>   	/*
>> @@ -287,6 +302,9 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>>   	slot->bus = pci_bus_get(parent);
>>   	slot->number = slot_nr;
>>   
>> +	if (pci_slot_enabled_per_func())
>> +		slot->per_func_slot = 1;
>> +
>>   	slot->kobj.kset = pci_slots_kset;
>>   
>>   	slot_name = make_slot_name(name);
>> @@ -307,8 +325,7 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>>   
>>   	down_read(&pci_bus_sem);
>>   	list_for_each_entry(dev, &parent->devices, bus_list)
>> -		if (slot_nr == PCI_SLOT_ALL_DEVICES ||
>> -		    PCI_SLOT(dev->devfn) == slot_nr)
>> +		if (pci_dev_matches_slot(dev, slot))
>>   			dev->slot = slot;
>>   	up_read(&pci_bus_sem);
>>   
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index 2c4454583c11..d58982aa8730 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -78,14 +78,18 @@
>>    * and, if ARI Forwarding is enabled, functions may appear to be on multiple
>>    * devices.
>>    */
>> -#define PCI_SLOT_ALL_DEVICES	0xfe
>> +#define PCI_SLOT_ALL_DEVICES	0xfeff
>> +
>> +/* Used to identify a slot as a placeholder */
>> +#define PCI_SLOT_PLACEHOLDER	-1
>>   
>>   /* pci_slot represents a physical slot */
>>   struct pci_slot {
>>   	struct pci_bus		*bus;		/* Bus this slot is on */
>>   	struct list_head	list;		/* Node in list of slots */
>>   	struct hotplug_slot	*hotplug;	/* Hotplug info (move here) */
>> -	unsigned char		number;		/* Device nr, or PCI_SLOT_ALL_DEVICES */
>> +	u16			number;		/* Device nr, or PCI_SLOT_ALL_DEVICES */
>> +	unsigned int		per_func_slot:1; /* Allow per function slot */

Hi Gerd,


> How about you introduce two additional single-bit flag members here for
> - placeholder, and
> - slot_all_devices
> and avoid creating an artifically wide number member.
>
> Eventually, this means that the special cases "placeholder-slot" and
> "bus-wide slot" should be broken out of pci_create_slot().
>
>>   	struct kobject		kobj;
>>   };
>>
> Hope this makes any sense? It almost makes me wonder if this should be
> handled with a pre-cursor patch to this...

I would like to avoid doing this as part of this series, and not 
increase it's scope too much. I do see your point about having separate 
flags to indicate a placeholder/slot_all_devices, but I think we would 
still need the special numbers unless we want to change pci_create_slot 
API to pass in flags.

Thanks

Farhan

>
> Thanks,
> Gerd