[PATCH v2] vfio/pci: print vfio-device syspath to fdinfo

[PATCH v2] vfio/pci: print vfio-device syspath to fdinfo

[Alex Mastro]

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>

Re: [PATCH v2] vfio/pci: print vfio-device syspath to fdinfo

[Jason Gunthorpe]

On Thu, Jul 24, 2025 at 04:29:53PM -0700, Alex Mastro wrote:
> 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.

??

> +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);
                                 ^^^^^^^^^^^^^

vdev->pdev == core_vdev->dev 

int vfio_pci_core_init_dev(struct vfio_device *core_vdev)
{
	vdev->pdev = to_pci_dev(core_vdev->dev);

It is the right thing to just make this generic and use
core_dev->dev.kobj

Every vfio parent device has a sysfs path.

Jason

Re: [PATCH v2] vfio/pci: print vfio-device syspath to fdinfo

[Alex Mastro]

On Fri, Aug 01, 2025 at 11:48:00AM -0300, Jason Gunthorpe wrote:
> ??
<...>
> vdev->pdev == core_vdev->dev 
<...>
> Every vfio parent device has a sysfs path.

Hi Jason, thanks for bearing with me. I lacked some fundamental understandings
here. This suggestion makes sense, and is simpler and more to the point. I'll
address this in v3, and also add the documentation for this new behavior to
proc.rst.