Re: [PATCH v6 2/2] PCI: s390: Expose the UID as an arch specific PCI slot attribute

[Farhan Ali <alifm@linux.ibm.com>]

On 4/2/2026 1:34 PM, Niklas Schnelle wrote:
> On s390, an individual PCI function can generally be identified by two
> identifiers, the FID and the UID. Which identifier is used depends on
> the scope and the platform configuration.
>
> The first identifier, the FID, is always available and identifies a PCI
> device uniquely within a machine. The FID may be virtualized by
> hypervisors, but on the LPAR level, the machine scope makes it
> impossible to create the same configuration based on FIDs on two
> different LPARs of the same machine, and difficult to reuse across
> machines.
>
> Such matching LPAR configurations are useful, though, allowing
> standardized setups and booting a Linux installation on different LPARs.
> To this end the UID, or user-defined identifier, was introduced. While
> it is only guaranteed to be unique within an LPAR and only if indicated
> by firmware, it allows users to replicate PCI device setups.
>
> On s390, which uses a machine hypervisor, a per PCI function hotplug
> model is used. The shortcoming with the UID then is, that it is not
> visible to the user without first attaching the PCI function and
> accessing the "uid" device attribute. The FID, on the other hand, is
> used as the slot name and is thus known even with the PCI function in
> standby.
>
> Remedy this shortcoming by providing the UID as an attribute on the slot
> allowing the user to identify a PCI function based on the UID without
> having to first attach it. Do this via a macro mechanism analogous to
> what was introduced by commit 265baca69a07 ("s390/pci: Stop usurping
> pdev->dev.groups") for the PCI device attributes.
>
> Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
> Reviewed-by: Julian Ruess <julianr@linux.ibm.com>
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
>   Documentation/arch/s390/pci.rst |  7 +++++++
>   arch/s390/include/asm/pci.h     |  4 ++++
>   arch/s390/pci/pci_sysfs.c       | 20 ++++++++++++++++++++
>   drivers/pci/slot.c              | 13 ++++++++++++-
>   4 files changed, 43 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/arch/s390/pci.rst b/Documentation/arch/s390/pci.rst
> index 31c24ed5506f1fc07f89821f67a814118514f441..4c0f35c8a5588eee3cf0d596e0057f24b3ed079c 100644
> --- a/Documentation/arch/s390/pci.rst
> +++ b/Documentation/arch/s390/pci.rst
> @@ -57,6 +57,13 @@ Entries specific to zPCI functions and entries that hold zPCI information.
>   
>     - /sys/bus/pci/slots/XXXXXXXX/power
>   
> +  In addition to using the FID as the name of the slot the slot directory
> +  also contains the following s390 specific slot attributes.
> +
> +  - uid:
> +    The User-defined identifier (UID) of the function which may be configured
> +    by this slot. See also the corresponding attribute of the device.
> +
>     A physical function that currently supports a virtual function cannot be
>     powered off until all virtual functions are removed with:
>     echo 0 > /sys/bus/pci/devices/DDDD:BB:dd.f/sriov_numvf
> diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
> index c0ff19dab5807c7e1aabb48a0e9436aac45ec97d..5dcf35f0f325f5f44b28109a1c8d9aef18401035 100644
> --- a/arch/s390/include/asm/pci.h
> +++ b/arch/s390/include/asm/pci.h
> @@ -208,6 +208,10 @@ extern const struct attribute_group zpci_ident_attr_group;
>   			    &pfip_attr_group,		 \
>   			    &zpci_ident_attr_group,
>   
> +extern const struct attribute_group zpci_slot_attr_group;
> +
> +#define ARCH_PCI_SLOT_GROUPS (&zpci_slot_attr_group)
> +
>   extern unsigned int s390_pci_force_floating __initdata;
>   extern unsigned int s390_pci_no_rid;
>   
> diff --git a/arch/s390/pci/pci_sysfs.c b/arch/s390/pci/pci_sysfs.c
> index c2444a23e26c4218832bb91930b5f0ffd498d28f..d98d97df792adb3c7e415a8d374cc2f3a65fbb52 100644
> --- a/arch/s390/pci/pci_sysfs.c
> +++ b/arch/s390/pci/pci_sysfs.c
> @@ -187,6 +187,17 @@ static ssize_t index_show(struct device *dev,
>   }
>   static DEVICE_ATTR_RO(index);
>   
> +static ssize_t zpci_uid_slot_show(struct pci_slot *slot, char *buf)
> +{
> +	struct zpci_dev *zdev = container_of(slot->hotplug, struct zpci_dev,
> +					     hotplug_slot);
> +
> +	return sysfs_emit(buf, "0x%x\n", zdev->uid);
> +}

I think the way we assign the same pci slot to multifunctions (PF and VF 
on the same LPAR), IIUC we could possibly display the wrong uid here. I 
am hoping my patch [1] can fix this. I am curious, is there any 
s390-tool that would use the uid? Though this would only impact the NETD 
devices on newer machines, so I don't know if we want to gate this 
behind my patch?

[1] https://lore.kernel.org/all/20260330174011.1161-2-alifm@linux.ibm.com/

Thanks

Farhan

> +
> +static struct pci_slot_attribute zpci_slot_attr_uid =
> +	__ATTR(uid, 0444, zpci_uid_slot_show, NULL);
> +
>   static umode_t zpci_index_is_visible(struct kobject *kobj,
>   				     struct attribute *attr, int n)
>   {
> @@ -243,6 +254,15 @@ const struct attribute_group pfip_attr_group = {
>   	.attrs = pfip_attrs,
>   };
>   
> +static struct attribute *zpci_slot_attrs[] = {
> +	&zpci_slot_attr_uid.attr,
> +	NULL,
> +};
> +
> +const struct attribute_group zpci_slot_attr_group = {
> +	.attrs = zpci_slot_attrs,
> +};
> +
>   static struct attribute *clp_fw_attrs[] = {
>   	&uid_checking_attr.attr,
>   	NULL,
> diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
> index 787311614e5b6ebb39e7284f9b9f205a0a684d6d..2f8fcfbbec24e73d0bb6e40fd04c05a94f518045 100644
> --- a/drivers/pci/slot.c
> +++ b/drivers/pci/slot.c
> @@ -96,7 +96,18 @@ static struct attribute *pci_slot_default_attrs[] = {
>   	&pci_slot_attr_cur_speed.attr,
>   	NULL,
>   };
> -ATTRIBUTE_GROUPS(pci_slot_default);
> +
> +static const struct attribute_group pci_slot_default_group = {
> +	.attrs = pci_slot_default_attrs,
> +};
> +
> +static const struct attribute_group *pci_slot_default_groups[] = {
> +	&pci_slot_default_group,
> +#ifdef ARCH_PCI_SLOT_GROUPS
> +	ARCH_PCI_SLOT_GROUPS,
> +#endif
> +	NULL,
> +};
>   
>   static const struct kobj_type pci_slot_ktype = {
>   	.sysfs_ops = &pci_slot_sysfs_ops,
>