Re: [PATCH v3 4/7] vhost-vsock: preserve vhost FD during CPR

On Fri, Jun 26, 2026 at 07:46:40PM +0300, Andrey Drobyshev wrote:
>During CPR (checkpoint-restore) migration the guest keeps running on the
>same host, so instead of reopening /dev/vhost-vsock on the destination,
>we should reuse the FD from the source.  The FD is saved in the CPR
>namespace (hash table) with cpr_save_fd() and then reclaimed on the
>target via cpr_find_fd().
>
>Since the key in CPR hash table is device ID, CPR needs a unique ID.
>Rather than make it mandatory for every vhost-vsock device, we add CPR
>migration blocker which only fires once we attempt CPR with ID-less
>vhost-vsock.
>
>vhost_dev_init() (and thus VHOST_SET_OWNER) still runs in realize() here.
>Deferring the ownership handoff to pre_save/post_load is done in a
>following patch.

So will this patch be bisectabale?

Thanks,
Stefano

>
>Signed-off-by: Andrey Drobyshev <[email protected]>
>---
> hw/virtio/vhost-vsock.c         | 50 ++++++++++++++++++++++++++++++---
> include/hw/virtio/vhost-vsock.h |  1 +
> 2 files changed, 47 insertions(+), 4 deletions(-)
>
>diff --git a/hw/virtio/vhost-vsock.c b/hw/virtio/vhost-vsock.c
>index fd7ffa88990..7eacb608d07 100644
>--- a/hw/virtio/vhost-vsock.c
>+++ b/hw/virtio/vhost-vsock.c
>@@ -21,6 +21,8 @@
> #include "hw/virtio/vhost-vsock.h"
> #include "monitor/monitor.h"
> #include "migration/cpr.h"
>+#include "migration/blocker.h"
>+#include "migration/misc.h"
>
> static void vhost_vsock_get_config(VirtIODevice *vdev, uint8_t *config)
> {
>@@ -141,6 +143,7 @@ static void vhost_vsock_device_realize(DeviceState *dev, Error **errp)
>     VHostVSockCommon *vvc = VHOST_VSOCK_COMMON(dev);
>     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>     VHostVSock *vsock = VHOST_VSOCK(dev);
>+    DeviceState *proxy = qdev_get_parent_bus(DEVICE(vsock))->parent;
>     int vhostfd;
>     int ret;
>
>@@ -155,23 +158,49 @@ static void vhost_vsock_device_realize(DeviceState *dev, Error **errp)
>         return;
>     }
>
>-    if (vsock->conf.vhostfd) {
>+    /*
>+     * Having a unique ID is mandatory for FD preservation during CPR
>+     * migration, thus we add migration blockers for CPR modes.
>+     */
>+    if (!proxy->id) {
>+        error_setg(&vsock->migration_blocker,
>+                   "vhost-vsock: device ID is required for CPR migration");
>+        if (migrate_add_blocker_modes(&vsock->migration_blocker,
>+                                      BIT(MIG_MODE_CPR_TRANSFER) |
>+                                      BIT(MIG_MODE_CPR_EXEC), errp) < 0) {
>+            return;
>+        }
>+    }
>+
>+    if (cpr_is_incoming()) {
>+        /* Reuse the fd handed over from the source QEMU. */
>+        if (!proxy->id) {
>+            error_setg(errp, "vhost-vsock: device ID is required for "
>+                       "CPR migration");
>+            goto err_blocker;
>+        }
>+        vhostfd = cpr_find_fd(proxy->id, 0);
>+        if (vhostfd < 0) {
>+            error_setg(errp, "vhost-vsock: could not find restored vhost FD");
>+            goto err_blocker;
>+        }
>+    } else if (vsock->conf.vhostfd) {
>         vhostfd = monitor_fd_param(monitor_cur(), vsock->conf.vhostfd, errp);
>         if (vhostfd == -1) {
>             error_prepend(errp, "vhost-vsock: unable to parse vhostfd: ");
>-            return;
>+            goto err_blocker;
>         }
>     } else {
>         vhostfd = open("/dev/vhost-vsock", O_RDWR);
>         if (vhostfd < 0) {
>             error_setg_file_open(errp, errno, "/dev/vhost-vsock");
>-            return;
>+            goto err_blocker;
>         }
>     }
>
>     if (!qemu_set_blocking(vhostfd, false, errp)) {
>         close(vhostfd);
>-        return;
>+        goto err_blocker;
>     }
>
>     vhost_vsock_common_realize(vdev);
>@@ -192,6 +221,11 @@ static void vhost_vsock_device_realize(DeviceState *dev, Error **errp)
>         goto err_vhost_dev;
>     }
>
>+    /* Register the fd for a future CPR after a fully successful realize */
>+    if (proxy->id) {
>+        cpr_save_fd(proxy->id, 0, vhostfd);
>+    }
>+
>     return;
>
> err_vhost_dev:
>@@ -199,16 +233,24 @@ err_vhost_dev:
>     vhost_dev_cleanup(&vvc->vhost_dev);
> err_virtio:
>     vhost_vsock_common_unrealize(vdev);
>+err_blocker:
>+    migrate_del_blocker(&vsock->migration_blocker);
> }
>
> static void vhost_vsock_device_unrealize(DeviceState *dev)
> {
>     VHostVSockCommon *vvc = VHOST_VSOCK_COMMON(dev);
>     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>+    VHostVSock *vsock = VHOST_VSOCK(dev);
>+    DeviceState *proxy = qdev_get_parent_bus(dev)->parent;
>
>     /* This will stop vhost backend if appropriate. */
>     vhost_vsock_set_status(vdev, 0);
>
>+    if (proxy->id) {
>+        cpr_delete_fd(proxy->id, 0);
>+    }
>+    migrate_del_blocker(&vsock->migration_blocker);
>     vhost_dev_cleanup(&vvc->vhost_dev);
>     vhost_vsock_common_unrealize(vdev);
> }
>diff --git a/include/hw/virtio/vhost-vsock.h b/include/hw/virtio/vhost-vsock.h
>index 84f4e727c70..5ebc63afc5a 100644
>--- a/include/hw/virtio/vhost-vsock.h
>+++ b/include/hw/virtio/vhost-vsock.h
>@@ -29,6 +29,7 @@ struct VHostVSock {
>     /*< private >*/
>     VHostVSockCommon parent;
>     VHostVSockConf conf;
>+    Error *migration_blocker;   /* set when the device has no ID */
>
>     /*< public >*/
> };
>-- 
>2.47.1
>