linux-kernel - [PATCH v4] vfio/pci: print vfio-device syspath to fdinfo

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Message-ID: <20250804-show-fdinfo-v4-1-96b14c5691b3@fb.com>
Date: Mon, 4 Aug 2025 12:44:31 -0700
From: Alex Mastro <amastro@...com>
To: Alex Williamson <alex.williamson@...hat.com>,
        Jonathan Corbet
	<corbet@....net>
CC: Jason Gunthorpe <jgg@...pe.ca>, Keith Busch <kbusch@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <kvm@...r.kernel.org>,
        Alex Mastro
	<amastro@...com>
Subject: [PATCH v4] vfio/pci: print vfio-device syspath to fdinfo

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@...com>
---
Changes in v4:
- Remove changes to vfio.h
- Link to v3: https://lore.kernel.org/r/20250801-show-fdinfo-v3-1-165dfcab89b9@fb.com
Changes in v3:
- Remove changes to vfio_pci.c
- Add section to Documentation/filesystems/proc.rst
- Link to v2: https://lore.kernel.org/all/20250724-show-fdinfo-v2-1-2952115edc10@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
---
 Documentation/filesystems/proc.rst | 14 ++++++++++++++
 drivers/vfio/vfio_main.c           | 20 ++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 2a17865dfe39..fc5ed3117834 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -2162,6 +2162,20 @@ DMA Buffer files
 where 'size' is the size of the DMA buffer in bytes. 'count' is the file count of
 the DMA buffer file. 'exp_name' is the name of the DMA buffer exporter.
 
+VFIO Device files
+~~~~~~~~~~~~~~~~
+
+::
+
+	pos:    0
+	flags:  02000002
+	mnt_id: 17
+	ino:    5122
+	vfio-device-syspath: /sys/devices/pci0000:e0/0000:e0:01.1/0000:e1:00.0/0000:e2:05.0/0000:e8:00.0
+
+where 'vfio-device-syspath' is the sysfs path corresponding to the VFIO device
+file.
+
 3.9	/proc/<pid>/map_files - Information about memory mapped files
 ---------------------------------------------------------------------
 This directory contains symbolic links which represent memory mapped files
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 1fd261efc582..37a39cee10ed 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -28,6 +28,7 @@
 #include <linux/pseudo_fs.h>
 #include <linux/rwsem.h>
 #include <linux/sched.h>
+#include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/stat.h>
 #include <linux/string.h>
@@ -1354,6 +1355,22 @@ 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)
+{
+	char *path;
+	struct vfio_device_file *df = filep->private_data;
+	struct vfio_device *device = df->device;
+
+	path = kobject_get_path(&device->dev->kobj, GFP_KERNEL);
+	if (!path)
+		return;
+
+	seq_printf(m, "vfio-device-syspath: /sys%s\n", path);
+	kfree(path);
+}
+#endif
+
 const struct file_operations vfio_device_fops = {
 	.owner		= THIS_MODULE,
 	.open		= vfio_device_fops_cdev_open,
@@ -1363,6 +1380,9 @@ 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)

---
base-commit: 4518e5a60c7fbf0cdff393c2681db39d77b4f87e
change-id: 20250801-show-fdinfo-ef109ca738cf

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