vfio: Dump migration features under debugfs

A debugfs directory was recently added for VFIO devices. Add a new
"features" file under the migration sub-directory to expose which
features the device supports.

Signed-off-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20250918121928.1921871-1-clg@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: legoater

Documentation/ABI/testing/debugfs-vfio

@@ -23,3 +23,9 @@ Contact:	Longfang Liu <liulongfang@huawei.com>
 Description:	Read the live migration status of the vfio device.
 		The contents of the state file reflects the migration state
 		relative to those defined in the vfio_device_mig_state enum
+
+What:		/sys/kernel/debug/vfio/<device>/migration/features
+Date:		Oct 2025
+KernelVersion:	6.18
+Contact:	Cédric Le Goater <clg@redhat.com>
+Description:	Read the migration features of the vfio device.

drivers/vfio/debugfs.c

@@ -58,6 +58,23 @@ static int vfio_device_state_read(struct seq_file *seq, void *data)
 	return 0;
 }
 
+static int vfio_device_features_read(struct seq_file *seq, void *data)
+{
+	struct device *vf_dev = seq->private;
+	struct vfio_device *vdev = container_of(vf_dev, struct vfio_device, device);
+
+	if (vdev->migration_flags & VFIO_MIGRATION_STOP_COPY)
+		seq_puts(seq, "stop-copy\n");
+	if (vdev->migration_flags & VFIO_MIGRATION_P2P)
+		seq_puts(seq, "p2p\n");
+	if (vdev->migration_flags & VFIO_MIGRATION_PRE_COPY)
+		seq_puts(seq, "pre-copy\n");
+	if (vdev->log_ops)
+		seq_puts(seq, "dirty-tracking\n");
+
+	return 0;
+}
+
 void vfio_device_debugfs_init(struct vfio_device *vdev)
 {
 	struct device *dev = &vdev->device;
@@ -72,6 +89,8 @@ void vfio_device_debugfs_init(struct vfio_device *vdev)
 							vdev->debug_root);
 		debugfs_create_devm_seqfile(dev, "state", vfio_dev_migration,
 					    vfio_device_state_read);
+		debugfs_create_devm_seqfile(dev, "features", vfio_dev_migration,
+					    vfio_device_features_read);
 	}
 }