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

On Fri, Jun 26, 2026 at 07:46:41PM +0300, Andrey Drobyshev wrote:
>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 <[email protected]>
>---
> 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).

Should we mention that on error `vhost_dev_cleanup()` must be called, or 
maybe should we call it in the new function if vhost_dev_init_features() 
fails?

Thanks,
Stefano

>+ *
>+ * 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
>