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

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Message-ID: <20250911183307.1910-4-alifm@linux.ibm.com>
Date: Thu, 11 Sep 2025 11:33:00 -0700
From: Farhan Ali <alifm@...ux.ibm.com>
To: linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Cc: alex.williamson@...hat.com, helgaas@...nel.org, alifm@...ux.ibm.com,
        schnelle@...ux.ibm.com, mjrosato@...ux.ibm.com
Subject: [PATCH v3 03/10] PCI: Allow per function PCI slots

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@...ux.ibm.com>
Signed-off-by: Farhan Ali <alifm@...ux.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