Re: [PATCH v2 1/2] PCI: Allow per function PCI slots

Bibo Mao
2025-10-28T01:14:21Z

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
>
>>
>> Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
>> Cc: stable@vger.kernel.org
>> Suggested-by: Niklas Schnelle <schnelle@linux.ibm.com>
>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>> ---
>> 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;
>> +}
>> +
> --- snip ---
>>
>> 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@linux.ibm.com>
>