vhost-user: Interface for migration state transfer

Add the interface for transferring the back-end's state during migration
as defined previously in vhost-user.rst.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20231016134243.68248-6-hreitz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/hw/virtio/vhost-backend.h

@@ -26,6 +26,18 @@ typedef enum VhostSetConfigType {
     VHOST_SET_CONFIG_TYPE_MIGRATION = 1,
 } VhostSetConfigType;
 
+typedef enum VhostDeviceStateDirection {
+    /* Transfer state from back-end (device) to front-end */
+    VHOST_TRANSFER_STATE_DIRECTION_SAVE = 0,
+    /* Transfer state from front-end to back-end (device) */
+    VHOST_TRANSFER_STATE_DIRECTION_LOAD = 1,
+} VhostDeviceStateDirection;
+
+typedef enum VhostDeviceStatePhase {
+    /* The device (and all its vrings) is stopped */
+    VHOST_TRANSFER_STATE_PHASE_STOPPED = 0,
+} VhostDeviceStatePhase;
+
 struct vhost_inflight;
 struct vhost_dev;
 struct vhost_log;
@@ -129,6 +141,15 @@ typedef int (*vhost_set_config_call_op)(struct vhost_dev *dev,
 
 typedef void (*vhost_reset_status_op)(struct vhost_dev *dev);
 
+typedef bool (*vhost_supports_device_state_op)(struct vhost_dev *dev);
+typedef int (*vhost_set_device_state_fd_op)(struct vhost_dev *dev,
+                                            VhostDeviceStateDirection direction,
+                                            VhostDeviceStatePhase phase,
+                                            int fd,
+                                            int *reply_fd,
+                                            Error **errp);
+typedef int (*vhost_check_device_state_op)(struct vhost_dev *dev, Error **errp);
+
 typedef struct VhostOps {
     VhostBackendType backend_type;
     vhost_backend_init vhost_backend_init;
@@ -176,6 +197,9 @@ typedef struct VhostOps {
     vhost_force_iommu_op vhost_force_iommu;
     vhost_set_config_call_op vhost_set_config_call;
     vhost_reset_status_op vhost_reset_status;
+    vhost_supports_device_state_op vhost_supports_device_state;
+    vhost_set_device_state_fd_op vhost_set_device_state_fd;
+    vhost_check_device_state_op vhost_check_device_state;
 } VhostOps;
 
 int vhost_backend_update_device_iotlb(struct vhost_dev *dev,

include/hw/virtio/vhost-user.h

@@ -31,6 +31,7 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_STATUS = 16,
     /* Feature 17 reserved for VHOST_USER_PROTOCOL_F_XEN_MMAP. */
     VHOST_USER_PROTOCOL_F_SHARED_OBJECT = 18,
+    VHOST_USER_PROTOCOL_F_DEVICE_STATE = 19,
     VHOST_USER_PROTOCOL_F_MAX
 };
 

include/hw/virtio/vhost.h

@@ -351,4 +351,82 @@ static inline int vhost_reset_device(struct vhost_dev *hdev)
 }
 #endif /* CONFIG_VHOST */
 
+/**
+ * vhost_supports_device_state(): Checks whether the back-end supports
+ * transferring internal device state for the purpose of migration.
+ * Support for this feature is required for vhost_set_device_state_fd()
+ * and vhost_check_device_state().
+ *
+ * @dev: The vhost device
+ *
+ * Returns true if the device supports these commands, and false if it
+ * does not.
+ */
+bool vhost_supports_device_state(struct vhost_dev *dev);
+
+/**
+ * vhost_set_device_state_fd(): Begin transfer of internal state from/to
+ * the back-end for the purpose of migration.  Data is to be transferred
+ * over a pipe according to @direction and @phase.  The sending end must
+ * only write to the pipe, and the receiving end must only read from it.
+ * Once the sending end is done, it closes its FD.  The receiving end
+ * must take this as the end-of-transfer signal and close its FD, too.
+ *
+ * @fd is the back-end's end of the pipe: The write FD for SAVE, and the
+ * read FD for LOAD.  This function transfers ownership of @fd to the
+ * back-end, i.e. closes it in the front-end.
+ *
+ * The back-end may optionally reply with an FD of its own, if this
+ * improves efficiency on its end.  In this case, the returned FD is
+ * stored in *reply_fd.  The back-end will discard the FD sent to it,
+ * and the front-end must use *reply_fd for transferring state to/from
+ * the back-end.
+ *
+ * @dev: The vhost device
+ * @direction: The direction in which the state is to be transferred.
+ *             For outgoing migrations, this is SAVE, and data is read
+ *             from the back-end and stored by the front-end in the
+ *             migration stream.
+ *             For incoming migrations, this is LOAD, and data is read
+ *             by the front-end from the migration stream and sent to
+ *             the back-end to restore the saved state.
+ * @phase: Which migration phase we are in.  Currently, there is only
+ *         STOPPED (device and all vrings are stopped), in the future,
+ *         more phases such as PRE_COPY or POST_COPY may be added.
+ * @fd: Back-end's end of the pipe through which to transfer state; note
+ *      that ownership is transferred to the back-end, so this function
+ *      closes @fd in the front-end.
+ * @reply_fd: If the back-end wishes to use a different pipe for state
+ *            transfer, this will contain an FD for the front-end to
+ *            use.  Otherwise, -1 is stored here.
+ * @errp: Potential error description
+ *
+ * Returns 0 on success, and -errno on failure.
+ */
+int vhost_set_device_state_fd(struct vhost_dev *dev,
+                              VhostDeviceStateDirection direction,
+                              VhostDeviceStatePhase phase,
+                              int fd,
+                              int *reply_fd,
+                              Error **errp);
+
+/**
+ * vhost_set_device_state_fd(): After transferring state from/to the
+ * back-end via vhost_set_device_state_fd(), i.e. once the sending end
+ * has closed the pipe, inquire the back-end to report any potential
+ * errors that have occurred on its side.  This allows to sense errors
+ * like:
+ * - During outgoing migration, when the source side had already started
+ *   to produce its state, something went wrong and it failed to finish
+ * - During incoming migration, when the received state is somehow
+ *   invalid and cannot be processed by the back-end
+ *
+ * @dev: The vhost device
+ * @errp: Potential error description
+ *
+ * Returns 0 when the back-end reports successful state transfer and
+ * processing, and -errno when an error occurred somewhere.
+ */
+int vhost_check_device_state(struct vhost_dev *dev, Error **errp);
+
 #endif