A few nits. Looks like there has been some churn here so this patch
will need to be rebased.
On Wed, Aug 13, 2025 at 1:01 PM Vladimir Sementsov-Ogievskiy
<vsementsov@yandex-team.ru> wrote:
>
> Add logic to transfer virtio notifiers through migration channel
> for vhost backend migration case.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> ---
> hw/virtio/virtio-bus.c | 2 +-
> hw/virtio/virtio.c | 74 ++++++++++++++++++++++++++++++++++++--
> include/hw/virtio/virtio.h | 2 ++
> 3 files changed, 75 insertions(+), 3 deletions(-)
>
> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> index c7e3941b1e..c1848144a2 100644
> --- a/hw/virtio/virtio-bus.c
> +++ b/hw/virtio/virtio-bus.c
> @@ -286,7 +286,7 @@ int virtio_bus_set_host_notifier(VirtioBusState *bus, int n, bool assign)
> return -ENOSYS;
> }
>
> - if (assign) {
> + if (assign && !virtio_is_vhost_migrating_backend(vdev)) {
> r = event_notifier_init(notifier, 1);
> if (r < 0) {
> error_report("%s: unable to init event notifier: %s (%d)",
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 10891f0e0c..87c243edad 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -26,6 +26,7 @@
> #include "hw/virtio/virtio.h"
> #include "hw/virtio/vhost.h"
> #include "migration/qemu-file-types.h"
> +#include "migration/qemu-file.h"
> #include "qemu/atomic.h"
> #include "hw/virtio/virtio-bus.h"
> #include "hw/qdev-properties.h"
> @@ -2992,6 +2993,7 @@ int virtio_save(VirtIODevice *vdev, QEMUFile *f)
> VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
> uint32_t guest_features_lo = (vdev->guest_features & 0xffffffff);
> int i;
> + bool migrating_backend = virtio_is_vhost_migrating_backend(vdev);
>
> if (k->save_config) {
> k->save_config(qbus->parent, f);
> @@ -3025,11 +3027,23 @@ int virtio_save(VirtIODevice *vdev, QEMUFile *f)
> */
> qemu_put_be64(f, vdev->vq[i].vring.desc);
> qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);
> +
> + if (migrating_backend) {
> + qemu_file_put_fd(f,
> + event_notifier_get_fd(&vdev->vq[i].host_notifier));
> + qemu_file_put_fd(
> + f, event_notifier_get_fd(&vdev->vq[i].guest_notifier));
> + }
> +
> if (k->save_queue) {
> k->save_queue(qbus->parent, i, f);
> }
> }
>
> + if (migrating_backend) {
> + qemu_file_put_fd(f, event_notifier_get_fd(&vdev->config_notifier));
> + }
> +
> if (vdc->save != NULL) {
> vdc->save(vdev, f);
> }
> @@ -3235,6 +3249,8 @@ virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
> BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
> VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
> + Error *local_err = NULL;
> + bool migrating_backend = virtio_is_vhost_migrating_backend(vdev);
>
> /*
> * We poison the endianness to ensure it does not get used before
> @@ -3304,6 +3320,13 @@ virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
> vdev->vq[i].signalled_used_valid = false;
> vdev->vq[i].notification = true;
>
> + if (migrating_backend) {
> + event_notifier_init_fd(&vdev->vq[i].host_notifier,
> + qemu_file_get_fd(f));
> + event_notifier_init_fd(&vdev->vq[i].guest_notifier,
> + qemu_file_get_fd(f));
> + }
> +
> if (!vdev->vq[i].vring.desc && vdev->vq[i].last_avail_idx) {
> error_report("VQ %d address 0x0 "
> "inconsistent with Host index 0x%x",
> @@ -3317,6 +3340,10 @@ virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
> }
> }
>
> + if (migrating_backend) {
nit: spurious spaces after &vdev->config_notifier
> + event_notifier_init_fd(&vdev->config_notifier , qemu_file_get_fd(f));
> + }
> +
> virtio_notify_vector(vdev, VIRTIO_NO_VECTOR);
>
> if (vdc->load != NULL) {
> @@ -3333,6 +3360,19 @@ virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
> }
> }
>
> + if (migrating_backend) {
> + /*
> + * On vhost backend migration, device do load host_features from
> + * migration stream. So update host_features.
> + */
> + vdev->host_features = vdc->get_features(vdev, vdev->host_features,
> + &local_err);
> + if (local_err) {
> + error_report_err(local_err);
> + return -EINVAL;
> + }
> + }
> +
> /* Subsections */
> ret = vmstate_load_state(f, &vmstate_virtio, vdev, 1);
> if (ret) {
> @@ -3394,6 +3434,18 @@ virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
> continue;
> }
>
> + if (migrating_backend) {
> + /*
"prior to backend migration"?
> + * Indices are not synced prior backend migration (as we don't
> + * stop vrings by GET_VRING_BASE). No reason to sync them now,
> + * and do any checks.
> + */
> + vdev->vq[i].used_idx = 0;
> + vdev->vq[i].shadow_avail_idx = 0;
> + vdev->vq[i].inuse = 0;
> + continue;
> + }
> +
> nheads = vring_avail_idx(&vdev->vq[i]) - vdev->vq[i].last_avail_idx;
> /* Check it isn't doing strange things with descriptor numbers. */
> if (nheads > vdev->vq[i].vring.num) {
> @@ -3762,8 +3814,9 @@ int virtio_queue_set_guest_notifier(VirtIODevice *vdev, int n, bool assign,
> EventNotifierHandler *read_fn = is_config ?
> virtio_config_guest_notifier_read :
> virtio_queue_guest_notifier_read;
> + bool migrating_backend = virtio_is_vhost_migrating_backend(vdev);
>
> - if (assign) {
> + if (assign && !migrating_backend) {
> int r = event_notifier_init(notifier, 0);
> if (r < 0) {
> return r;
> @@ -3773,7 +3826,7 @@ int virtio_queue_set_guest_notifier(VirtIODevice *vdev, int n, bool assign,
> event_notifier_set_handler(notifier,
> (assign && !with_irqfd) ? read_fn : NULL);
>
> - if (!assign) {
> + if (!assign && !migrating_backend) {
> /* Test and clear notifier before closing it,*/
> /* in case poll callback didn't have time to run. */
> read_fn(notifier);
> @@ -4392,6 +4445,23 @@ done:
> return element;
> }
>
> +bool virtio_is_vhost_migrating_backend(VirtIODevice *vdev)
> +{
> + VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
> + struct vhost_dev *hdev;
> +
> + if (!vdc->get_vhost) {
> + return false;
> + }
> +
> + hdev = vdc->get_vhost(vdev);
> + if (!hdev) {
> + return false;
> + }
> +
> + return hdev->migrating_backend;
> +}
> +
> static const TypeInfo virtio_device_info = {
> .name = TYPE_VIRTIO_DEVICE,
> .parent = TYPE_DEVICE,
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index 9a4a0a94aa..f94a7e5895 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -238,6 +238,8 @@ struct VirtioDeviceClass {
> bool (*skip_vhost_migration_log)(VirtIODevice *vdev);
> };
>
> +bool virtio_is_vhost_migrating_backend(VirtIODevice *vdev);
> +
> void virtio_instance_init_common(Object *proxy_obj, void *data,
> size_t vdev_size, const char *vdev_name);
>
> --
> 2.48.1
>
>