Transitioning from VFIO_DEVICE_STATE_STOP to VFIO_DEVICE_STATE_RESUMING
starts a process of restoring the device state of the vfio device on the
target system. To prepare for restoring the device state, open a file
stream to receive the guest's AP configuration saved when the device state
on the source system transitioned from VFIO_DEVICE_STATE_STOP to
VFIO_DEVICE_STATE_STOP_COPY.

Note:
This commit introduces the file operations structure for migration state
files with a stub implementation of the write 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>
Reviewed-by: Jason J. Herne <jjherne@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 dc1d951a8479..99062dcee01e 100644
--- a/drivers/s390/crypto/vfio_ap_migration.c
+++ b/drivers/s390/crypto/vfio_ap_migration.c
@@ -13,6 +13,16 @@
 #define VFIO_AP_MIG_MAGIC			0x76666170U  /* "vfap" */
 #define VFIO_AP_MIG_VERSION			1U
 
+/* The maximum number of queues that can be installed in an s390 system */
+#define MAX_AP_QUEUES				(AP_DEVICES * AP_DOMAINS)
+
+/* The maximum size of a vfio_ap_config blob, used to pre-allocate the receive
+ * buffer on the destination host during the RESUMING phase of migration.
+ */
+#define VFIO_AP_CONFIG_MAX_SIZE \
+	(sizeof(struct vfio_ap_config) + \
+	 MAX_AP_QUEUES * sizeof(struct vfio_ap_queue_info))
+
 /**
  * struct vfio_ap_migration_file:
  *
@@ -456,6 +466,57 @@ static struct file *vfio_ap_open_file_stream(struct vfio_ap_migration_file *mig_
 	return filp;
 }
 
+static ssize_t vfio_ap_resuming_write(struct file *filp, const char __user *buf,
+				      size_t len, loff_t *pos)
+{
+	/* TODO */
+	return -EOPNOTSUPP;
+}
+
+static const struct file_operations vfio_ap_resume_fops = {
+	.owner = THIS_MODULE,
+	.write = vfio_ap_resuming_write,
+	.release = vfio_ap_release_mig_file,
+};
+
+/**
+ * vfio_ap_resuming_init
+ *
+ * Initializes the ap_config, config_sz and filp fields of a
+ * struct vfio_ap_migration_file instance. Pre-allocates the maximum possible
+ * receive buffer using kvzalloc so that vfio_ap_resuming_write() can copy
+ * directly into it without needing to know num_queues up front. kvzalloc falls
+ * back to vmalloc for large allocations rather than failing a high-order
+ * kmalloc.
+ *
+ * @mig_file:	pointer to object that maintains the state of the migration
+ *		data stream.
+ */
+static struct file *vfio_ap_resuming_init(struct vfio_ap_migration_file *mig_file)
+{
+	void *ap_config;
+	struct file *filp;
+
+	lockdep_assert_held(&matrix_dev->mdevs_lock);
+
+	ap_config = kvzalloc(VFIO_AP_CONFIG_MAX_SIZE, GFP_KERNEL_ACCOUNT);
+	if (!ap_config)
+		return ERR_PTR(-ENOMEM);
+
+	filp = vfio_ap_open_file_stream(mig_file, &vfio_ap_resume_fops, O_WRONLY);
+
+	if (IS_ERR(filp)) {
+		kvfree(ap_config);
+		return filp;
+	}
+
+	mig_file->ap_config = ap_config;
+	mig_file->config_sz = VFIO_AP_CONFIG_MAX_SIZE;
+	mig_file->filp = filp;
+
+	return filp;
+}
+
 static struct file *
 vfio_ap_transition_to_state(struct ap_matrix_mdev *matrix_mdev,
 			    enum vfio_device_mig_state new_state)
@@ -488,10 +549,19 @@ vfio_ap_transition_to_state(struct ap_matrix_mdev *matrix_mdev,
 		return filp;
 	}
 
+	/*
+	 * Begins the process of restoring the vfio device state by creating and
+	 * returning a streaming data_fd to be used to read in the internal
+	 * state of the vfio-ap device on the destination host.
+	 */
 	if (cur_state == VFIO_DEVICE_STATE_STOP &&
 	    new_state == VFIO_DEVICE_STATE_RESUMING) {
-		/* TODO */
-		return ERR_PTR(-EOPNOTSUPP);
+		struct file *filp = vfio_ap_resuming_init(&mig_data->resuming_mig_file);
+
+		if (IS_ERR(filp))
+			return ERR_CAST(filp);
+
+		return filp;
 	}
 
 	if ((cur_state == VFIO_DEVICE_STATE_RESUMING &&
-- 
2.53.0