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

[Niklas Schnelle]

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.

> 
> 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;
> +}
> +
--- 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@xxxxxxxxxxxxx>