Print the PCI device syspath to a vfio device's fdinfo. This enables tools
to query which device is associated with a given vfio device fd.

This results in output like below:

$ cat /proc/"$SOME_PID"/fdinfo/"$VFIO_FD" | grep vfio
vfio-device-syspath: /sys/devices/pci0000:e0/0000:e0:01.1/0000:e1:00.0/0000:e2:05.0/0000:e8:00.0

Signed-off-by: Alex Mastro <amastro@fb.com>
---
Based on the feedback received from Jason G. and Alex W. in v1, print
the PCI device syspath to fdinfo instead of the BDF. This provides a
more specific waypoint for user space to query for any other relevant
information about the device.

There was discussion about removing vfio_device_ops callbacks, and
relying on just dev_name() in vfio_main.c, but since the concept of PCI
syspath is implementation-specific, I think we need to keep some form
of callback.

Signed-off-by: Alex Mastro <amastro@fb.com>

Changes in v2:
- Instead of PCI bdf, print the fully-qualified syspath (prefixed by
  /sys) to fdinfo.
- Rename the field to "vfio-device-syspath". The term "syspath" was
  chosen for consistency e.g. libudev's usage of the term.
- Link to v1: https://lore.kernel.org/r/20250623-vfio-fdinfo-v1-1-c9cec65a2922@fb.com
---
 drivers/vfio/pci/vfio_pci.c | 21 +++++++++++++++++++++
 drivers/vfio/vfio_main.c    | 14 ++++++++++++++
 include/linux/vfio.h        |  2 ++
 3 files changed, 37 insertions(+)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 5ba39f7623bb..bf3e7d873990 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -17,10 +17,12 @@
 #include <linux/file.h>
 #include <linux/interrupt.h>
 #include <linux/iommu.h>
+#include <linux/kobject.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/notifier.h>
 #include <linux/pm_runtime.h>
+#include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/types.h>
 #include <linux/uaccess.h>
@@ -125,6 +127,22 @@ static int vfio_pci_open_device(struct vfio_device *core_vdev)
 	return 0;
 }
 
+#ifdef CONFIG_PROC_FS
+static void vfio_pci_core_show_fdinfo(struct vfio_device *core_vdev, struct seq_file *m)
+{
+	char *path;
+	struct vfio_pci_core_device *vdev =
+		container_of(core_vdev, struct vfio_pci_core_device, vdev);
+
+	path = kobject_get_path(&vdev->pdev->dev.kobj, GFP_KERNEL);
+	if (!path)
+		return;
+
+	seq_printf(m, "vfio-device-syspath: /sys%s\n", path);
+	kfree(path);
+}
+#endif
+
 static const struct vfio_device_ops vfio_pci_ops = {
 	.name		= "vfio-pci",
 	.init		= vfio_pci_core_init_dev,
@@ -138,6 +156,9 @@ static const struct vfio_device_ops vfio_pci_ops = {
 	.mmap		= vfio_pci_core_mmap,
 	.request	= vfio_pci_core_request,
 	.match		= vfio_pci_core_match,
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo	= vfio_pci_core_show_fdinfo,
+#endif
 	.bind_iommufd	= vfio_iommufd_physical_bind,
 	.unbind_iommufd	= vfio_iommufd_physical_unbind,
 	.attach_ioas	= vfio_iommufd_physical_attach_ioas,
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 1fd261efc582..6e883c0c320b 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -1354,6 +1354,17 @@ static int vfio_device_fops_mmap(struct file *filep, struct vm_area_struct *vma)
 	return device->ops->mmap(device, vma);
 }
 
+#ifdef CONFIG_PROC_FS
+static void vfio_device_show_fdinfo(struct seq_file *m, struct file *filep)
+{
+	struct vfio_device_file *df = filep->private_data;
+	struct vfio_device *device = df->device;
+
+	if (device->ops->show_fdinfo)
+		device->ops->show_fdinfo(device, m);
+}
+#endif
+
 const struct file_operations vfio_device_fops = {
 	.owner		= THIS_MODULE,
 	.open		= vfio_device_fops_cdev_open,
@@ -1363,6 +1374,9 @@ const struct file_operations vfio_device_fops = {
 	.unlocked_ioctl	= vfio_device_fops_unl_ioctl,
 	.compat_ioctl	= compat_ptr_ioctl,
 	.mmap		= vfio_device_fops_mmap,
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo	= vfio_device_show_fdinfo,
+#endif
 };
 
 static struct vfio_device *vfio_device_from_file(struct file *file)
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 707b00772ce1..54076045a44f 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -16,6 +16,7 @@
 #include <linux/cdev.h>
 #include <uapi/linux/vfio.h>
 #include <linux/iova_bitmap.h>
+#include <linux/seq_file.h>
 
 struct kvm;
 struct iommufd_ctx;
@@ -135,6 +136,7 @@ struct vfio_device_ops {
 	void	(*dma_unmap)(struct vfio_device *vdev, u64 iova, u64 length);
 	int	(*device_feature)(struct vfio_device *device, u32 flags,
 				  void __user *arg, size_t argsz);
+	void	(*show_fdinfo)(struct vfio_device *device, struct seq_file *m);
 };
 
 #if IS_ENABLED(CONFIG_IOMMUFD)

---
base-commit: c1d9dac0db168198b6f63f460665256dedad9b6e
change-id: 20250724-show-fdinfo-9a916c6779f5

Best regards,
-- 
Alex Mastro <amastro@fb.com>