[PATCH v11 06/14] s390/vfio-ap: Transition guest migration state from STOP to STOP_COPY

[Anthony Krowiak <akrowiak@linux.ibm.com>]

The transition of the guest migration state from VFIO_DEVICE_STATE_STOP to
VFIO_DEVICE_STATE_STOP_COPY begins the process of saving the
vfio device state. The vfio device state is comprised of the guest's
AP configuration which specifies the adapters, domains and control
domains to which the guest will be given access.

To begin the process of saving the vfio device state, the VFIO AP device
driver opens a file stream that will be used to read the source guest's
AP configuration.

Note:
This commit introduces the file operations structure for migration state
files with a stub implementation of the read operation. The complete
implementation is provided in the subsequent commit.

This incremental approach allows for easier review of the migration
infrastructure setup separate from the actual implementation logic.

Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
---
 drivers/s390/crypto/vfio_ap_migration.c | 74 ++++++++++++++++++++++++-
 1 file changed, 72 insertions(+), 2 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_migration.c b/drivers/s390/crypto/vfio_ap_migration.c
index 8e4e04e03965..48028b12b26f 100644
--- a/drivers/s390/crypto/vfio_ap_migration.c
+++ b/drivers/s390/crypto/vfio_ap_migration.c
@@ -4,6 +4,7 @@
  *
  * Copyright IBM Corp. 2025
  */
+#include <linux/anon_inodes.h>
 #include <linux/file.h>
 #include "vfio_ap_private.h"
 
@@ -143,6 +144,62 @@ static void vfio_ap_release_resuming_file(struct vfio_ap_migration_data *mig_dat
 	mig_data->resuming_mig_data_complete = false;
 }
 
+static ssize_t
+vfio_ap_stop_copy_read(struct file *filp, char __user *buf, size_t len,
+		       loff_t *pos)
+{
+	/* TODO */
+	return -EOPNOTSUPP;
+}
+
+static int vfio_ap_release_mig_file(struct inode *file_inode, struct file *filp)
+{
+	struct vfio_ap_migration_file *mig_file = filp->private_data;
+
+	/*
+	 * The mig_file (and by extension mig_file->matrix_mdev) is tied to
+	 * migration FD lifetime (see vfio_ap_open_file_stream() function) to
+	 * guarantee that the matrix_mdev remains valid for every read/write
+	 * operation. Let's drop it here since the migration FD is being
+	 * released.
+	 */
+	vfio_device_put_registration(&mig_file->matrix_mdev->vdev);
+	return 0;
+}
+
+static const struct file_operations vfio_ap_stop_copy_fops = {
+	.owner = THIS_MODULE,
+	.read = vfio_ap_stop_copy_read,
+	.release = vfio_ap_release_mig_file,
+};
+
+static struct file *vfio_ap_open_file_stream(struct vfio_ap_migration_file *mig_file,
+					     const struct file_operations *fops,
+					     int flags)
+{
+	struct file *filp;
+
+	lockdep_assert_held(&matrix_dev->mdevs_lock);
+
+	/*
+	 * Pin the vfio_device registration so that matrix_mdev cannot be freed
+	 * while the migration FD is still open. The matching put is in
+	 * vfio_ap_release_mig_file().
+	 */
+	if (!vfio_device_try_get_registration(&mig_file->matrix_mdev->vdev))
+		return ERR_PTR(-ENODEV);
+
+	filp = anon_inode_getfile("vfio_ap_mig_file", fops, mig_file, flags);
+	if (IS_ERR(filp)) {
+		vfio_device_put_registration(&mig_file->matrix_mdev->vdev);
+		return filp;
+	}
+
+	stream_open(filp->f_inode, filp);
+
+	return filp;
+}
+
 static struct file *
 vfio_ap_transition_to_state(struct ap_matrix_mdev *matrix_mdev,
 			    enum vfio_device_mig_state new_state)
@@ -156,10 +213,23 @@ vfio_ap_transition_to_state(struct ap_matrix_mdev *matrix_mdev,
 	dev_dbg(matrix_mdev->vdev.dev, "%s: %d -> %d\n", __func__, cur_state,
 		new_state);
 
+	/*
+	 * Begins the process of saving the vfio device state by creating and
+	 * returning a streaming data_fd to be used to read out the internal
+	 * state of the vfio-ap device on the source host.
+	 */
 	if (cur_state == VFIO_DEVICE_STATE_STOP &&
 	    new_state == VFIO_DEVICE_STATE_STOP_COPY) {
-		/* TODO */
-		return ERR_PTR(-EOPNOTSUPP);
+		struct vfio_ap_migration_file *mig_file = &mig_data->stop_copy_mig_file;
+		struct file *filp = vfio_ap_open_file_stream(mig_file,
+							     &vfio_ap_stop_copy_fops,
+							     O_RDONLY);
+		if (IS_ERR(filp))
+			return ERR_CAST(filp);
+
+		mig_file->filp = filp;
+
+		return filp;
 	}
 
 	if (cur_state == VFIO_DEVICE_STATE_STOP &&
-- 
2.53.0