Re: [PATCH v2 1/2] PCI: Allow per function PCI slots
From: Bibo Mao
Date: Mon Oct 27 2025 - 21:14:21 EST
On 2025/10/27 下午8:29, Niklas Schnelle wrote:
On Wed, 2025-10-22 at 14:24 -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. 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.
I wonder if LoongArch which now also does per PCI function pass-through
might need this too. Adding their KVM maintainers.
Hi Niklas,
Thanks for your reminder. Yes, LoongArch do per PCI function pass-throught. In theory, function pci_slot_enabled_per_func() should return true on LoongArch also. Only that now IOMMU driver is not merged, there is no way to test it, however we will write down this as a note inside about this issue and verify it once IOMMU driver is merged.
Regards
Bibo Mao
--- snip ---
Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
Signed-off-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
---
drivers/pci/pci.c | 5 +++--
drivers/pci/slot.c | 25 ++++++++++++++++++++++---
include/linux/pci.h | 1 +
3 files changed, 26 insertions(+), 5 deletions(-)
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b14dd064006c..36ee38e0d817 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4980,8 +4980,9 @@ 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 50fb3eb595fe..ed10fa3ae727 100644
--- a/drivers/pci/slot.c
+++ b/drivers/pci/slot.c
@@ -63,6 +63,22 @@ 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 bool pci_slot_enabled_per_func(void)
+{
+ if (IS_ENABLED(CONFIG_S390))
+ return true;
+
+ return false;
+}
+
diff --git a/include/linux/pci.h b/include/linux/pci.h
index d1fdf81fbe1e..6ad194597ab5 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;
};
Reviewed-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>