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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 19, 2026 at 05:38:46PM +0300, Andrey Drobyshev wrote:
> On 8/18/26 6:29 PM, Stefano Garzarella wrote:
> > 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
> > 
> 
> The issue is that cpr-exec already is broken on the current master
> branch (i.e. before the series is applied).  The only difference is
> which exact error we fail with: -EBUSY or -EADDRINUSE.
> 
> In this case I don't think it's feasible to make each and every commit
> bisectable - it only becomes fully working after the last one applied.

Surely just bypassing cpr things until the last commit
is feasible?

> If you want, I can prepend the series with a migration blocker which
> would fail early and gracefully instead of crashing, and lift it after
> it's working.
> 
> Andrey
> 
> >>
> >> Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> >> ---
> >> 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
> >>
> >