[PATCH v3 5/7] vhost: factor out vhost_dev_init_backend()

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

Split the first part of vhost_dev_init(): selecting the backend, calling
its .vhost_init() and reading the supported features - into a new
vhost_dev_init_backend() helper, and call it from vhost_dev_init().

This is in preparation for CPR restore of vhost-vsock, which needs to learn
the backend's features at realize time to negotiate them when loading the
incoming virtio state, but also must defer taking ownership of the device
to post_load.  vhost_dev_init_backend() does exactly the pre-ownership part.

As a result VHOST_SET_OWNER now follows the feature query rather than
precedes it.  This should be safe, as no backend requires ownership before
VHOST_GET_FEATURES - the kernel and vdpa backends do not check ownership
for it, and vhost-user already does query features from its .vhost_init()
before set_owner().

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 hw/virtio/vhost.c         | 33 +++++++++++++++++++++++----------
 include/hw/virtio/vhost.h | 19 +++++++++++++++++++
 2 files changed, 42 insertions(+), 10 deletions(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index af41841b529..f11588cc51a 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1667,6 +1667,28 @@ static int vhost_dev_init_features(struct vhost_dev *hdev)
     return r;
 }
 
+int vhost_dev_init_backend(struct vhost_dev *hdev, void *opaque,
+                           VhostBackendType backend_type, Error **errp)
+{
+    int r;
+
+    r = vhost_set_backend_type(hdev, backend_type);
+    assert(r >= 0);
+
+    r = hdev->vhost_ops->vhost_init(hdev, opaque, errp);
+    if (r < 0) {
+        return r;
+    }
+
+    r = vhost_dev_init_features(hdev);
+    if (r < 0) {
+        error_setg_errno(errp, -r, "vhost_init_features failed");
+        return r;
+    }
+
+    return 0;
+}
+
 int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
                    VhostBackendType backend_type, uint32_t busyloop_timeout,
                    Error **errp)
@@ -1679,10 +1701,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
     hdev->vdev = NULL;
     hdev->migration_blocker = NULL;
 
-    r = vhost_set_backend_type(hdev, backend_type);
-    assert(r >= 0);
-
-    r = hdev->vhost_ops->vhost_init(hdev, opaque, errp);
+    r = vhost_dev_init_backend(hdev, opaque, backend_type, errp);
     if (r < 0) {
         goto fail;
     }
@@ -1693,12 +1712,6 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
         goto fail;
     }
 
-    r = vhost_dev_init_features(hdev);
-    if (r < 0) {
-        error_setg_errno(errp, -r, "vhost_init_features failed");
-        goto fail;
-    }
-
     limit = hdev->vhost_ops->vhost_memslots_limit(hdev);
     if (limit < MEMORY_DEVICES_SAFE_MAX_MEMSLOTS &&
         memory_devices_memslot_auto_decision_active()) {
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 684bafcaadd..bc81e09663e 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -156,6 +156,25 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
                    VhostBackendType backend_type,
                    uint32_t busyloop_timeout, Error **errp);
 
+/**
+ * vhost_dev_init_backend() - set up the backend and query its features
+ * @hdev: the common vhost_dev structure
+ * @opaque: opaque ptr passed to backend (vhost/vhost-user/vdpa)
+ * @backend_type: type of backend
+ * @errp: error handle
+ *
+ * Select the backend, initialise the backend instance and read its supported
+ * features into @hdev, without issuing VHOST_SET_OWNER, setting up the
+ * virtqueues or registering the memory listener.  This is the part of
+ * vhost_dev_init() that precedes taking ownership; it can be used on its own
+ * so feature negotiation can happen before ownership is acquired (e.g. by CPR
+ * restore).
+ *
+ * Return: 0 on success, non-zero on error while setting errp.
+ */
+int vhost_dev_init_backend(struct vhost_dev *hdev, void *opaque,
+                           VhostBackendType backend_type, Error **errp);
+
 /**
  * vhost_dev_cleanup() - tear down and cleanup vhost interface
  * @hdev: the common vhost_dev structure
-- 
2.47.1