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

Gerd Bayer
2026-05-04T15:52:48Z

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 */

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...

Thanks,
Gerd