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

[Gerd Bayer <gbayer@linux.ibm.com>]

On Mon, 2026-05-04 at 10:00 -0700, Farhan Ali wrote:
> 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(-)
> > > 
> > > 
> > > 

[... snip ...]

> > > 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.

I think I found a way to keep that untouched with some additional
changes as a pre-cursor: Below, you can find what I hacked together
extending on your idea of introducing flags for the slots. That way I
was able to decouple the input parameter slot_nr (int) from the actual
slot->number member (u8). 
Caveat: I *did* change PCI_SLOT_ALL_DEVICES (similar like you) - and
this is compile-tested only - and lacks all updates to comments.

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/slot.c b/drivers/pci/slot.c
index 6d5cd37bfb1e..b3d54197e8c9 100644
--- a/drivers/pci/slot.c
+++ b/drivers/pci/slot.c
@@ -37,20 +37,11 @@ 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->placeholder)
 		return sysfs_emit(buf, "%04x:%02x\n",
 				  pci_domain_nr(slot->bus),
 				  slot->bus->number);
 
-	/*
-	 * Preserve legacy ABI expectations that hotplug drivers that
manage
-	 * multiple devices per slot emit 0 for the device number.
-	 */
-	if (slot->number == PCI_SLOT_ALL_DEVICES)
-		return sysfs_emit(buf, "%04x:%02x:00\n",
-				  pci_domain_nr(slot->bus),
-				  slot->bus->number);
-
 	return sysfs_emit(buf, "%04x:%02x:%02x\n",
 			  pci_domain_nr(slot->bus),
 			  slot->bus->number,
@@ -82,7 +73,7 @@ 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 ||
+		if (slot->bus_wide ||
 		    PCI_SLOT(dev->devfn) == slot->number)
 			dev->slot = NULL;
 	up_read(&pci_bus_sem);
@@ -187,7 +178,7 @@ 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 ||
+		if (slot->bus_wide  ||
 		    PCI_SLOT(dev->devfn) == slot->number)
 			dev->slot = slot;
 	mutex_unlock(&pci_slot_mutex);
@@ -267,7 +258,7 @@ 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;
 
 	/*
@@ -296,7 +287,12 @@ struct pci_slot *pci_create_slot(struct pci_bus
*parent, int slot_nr,
 	}
 
 	slot->bus = pci_bus_get(parent);
-	slot->number = slot_nr;
+	if (slot_nr == PCI_SLOT_PLACEHOLDER)
+		slot->placeholder = 1;
+	else if (slot_nr == PCI_SLOT_ALL_DEVICES)
+		slot->bus_wide = 1;
+	else
+		slot->number = slot_nr;
 
 	slot->kobj.kset = pci_slots_kset;
 
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 2c4454583c11..9a27fddeb397 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -78,14 +78,17 @@
  * 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	-2
+#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 */
+	unsigned char		number;		/* Device nr
*/
+	unsigned int		bus_wide:1;	/* created with
PCI_SLOT_ALL_DEVICES */
+	unsigned int		placeholder:1;	/* special case for
PPC */
 	struct kobject		kobj;
 };
 

Nice side-effect: The special handling for bus-wide slots in
address_read_file() is no longer necessary.

Adding Keith Busch who introduced "bus-wide slots" just recently with
102c8b26b54e ("PCI: Allow all bus devices to use the same slot")
which actually added special meanings for slot 0xfe - which is not
always desired.

Maybe it is time to refactor pci_create_slot() into separate variants
for placeholder and bus-wide (and multifunction) slots... But that's a
bigger change that is out of scope here.

Thanks,
Gerd