[PATCH v3 03/10] PCI: Allow per function PCI slots

Farhan Ali
2025-09-11T18:33:16Z

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")
Suggested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
---
drivers/pci/hotplug/s390_pci_hpc.c | 10 ++++++++--
drivers/pci/pci.c | 4 +++-
drivers/pci/slot.c | 14 +++++++++++---
include/linux/pci.h | 1 +
4 files changed, 23 insertions(+), 6 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;
+ return 0;
}

void zpci_exit_slot(struct zpci_dev *zdev)
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 3994fa82df68..70296d3b1cfc 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -5061,7 +5061,9 @@ 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 ||
+ if (dev->multifunction && !dev->slot->per_func_slot)
+ return -ENOTTY;
+ if (dev->subordinate || !dev->slot ||
dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET)
return -ENOTTY;

diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
index 50fb3eb595fe..51ee59e14393 100644
--- a/drivers/pci/slot.c
+++ b/drivers/pci/slot.c
@@ -63,6 +63,14 @@ 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 PCI_SLOT(dev->devfn) == slot->number;
+}
+
static void pci_slot_release(struct kobject *kobj)
{
struct pci_dev *dev;
@@ -73,7 +81,7 @@ static void pci_slot_release(struct kobject *kobj)

down_read(&pci_bus_sem);
list_for_each_entry(dev, &slot->bus->devices, bus_list)
- if (PCI_SLOT(dev->devfn) == slot->number)
+ if (pci_dev_matches_slot(dev, slot))
dev->slot = NULL;
up_read(&pci_bus_sem);

@@ -166,7 +174,7 @@ void pci_dev_assign_slot(struct pci_dev *dev)

mutex_lock(&pci_slot_mutex);
list_for_each_entry(slot, &dev->bus->slots, list)
- if (PCI_SLOT(dev->devfn) == slot->number)
+ if (pci_dev_matches_slot(dev, slot))
dev->slot = slot;
mutex_unlock(&pci_slot_mutex);
}
@@ -285,7 +293,7 @@ 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 (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 59876de13860..9265f32d9786 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -78,6 +78,7 @@ struct pci_slot {
struct list_head list; /* Node in list of slots */
struct hotplug_slot *hotplug; /* Hotplug info (move here) */
unsigned char number; /* PCI_SLOT(pci_dev->devfn) */
+ unsigned int per_func_slot:1; /* Allow per function slot */
struct kobject kobj;
};

--
2.43.0