Re: [PATCH v6 4/5] vhost: add vmstate for inflight region with inner buffer

[Peter Xu <peterx@redhat.com>]

On Tue, Jan 13, 2026 at 02:58:17PM +0500, Alexandr Moshkov wrote:
> Prepare for future inflight region migration for vhost-user-blk.
> We need to migrate size, queue_size, and inner buffer.
> 
> So firstly it migrate size and queue_size fields, then allocate memory for buffer with
> migrated size, then migrate inner buffer itself.
> 
> Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
> ---
>  hw/virtio/vhost.c         | 57 +++++++++++++++++++++++++++++++++++++++
>  include/hw/virtio/vhost.h |  6 +++++
>  2 files changed, 63 insertions(+)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index c46203eb9c..f655c53b67 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -2028,6 +2028,63 @@ const VMStateDescription vmstate_backend_transfer_vhost_inflight = {
>      }
>  };
>  
> +static int vhost_inflight_buffer_pre_load(void *opaque, Error **errp)
> +{
> +    info_report("vhost_inflight_region_buffer_pre_load");
> +    struct vhost_inflight *inflight = opaque;
> +
> +    int fd = -1;
> +    void *addr = qemu_memfd_alloc("vhost-inflight", inflight->size,
> +                                  F_SEAL_GROW | F_SEAL_SHRINK | F_SEAL_SEAL,
> +                                  &fd, errp);
> +    if (*errp) {
> +        return -ENOMEM;
> +    }
> +
> +    inflight->offset = 0;
> +    inflight->addr = addr;
> +    inflight->fd = fd;
> +
> +    return 0;
> +}
> +
> +const VMStateDescription vmstate_vhost_inflight_region_buffer = {
> +    .name = "vhost-inflight-region/buffer",
> +    .pre_load_errp = vhost_inflight_buffer_pre_load,
> +    .fields = (const VMStateField[]) {
> +        VMSTATE_VBUFFER_UINT64(addr, struct vhost_inflight, 0, NULL, size),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
> +static int vhost_inflight_region_post_load(void *opaque,
> +                                           int version_id,
> +                                           Error **errp)
> +{
> +    struct vhost_inflight *inflight = opaque;
> +
> +    if (inflight->addr == NULL) {

IIUC this can never happen because pre_load() must trigger before
post_load(), and when reaching post_load() it means pre_load() must have
succeeded..

So, IIUC we can drop this post_load() completely (or assert addr in
pre_load instead).

> +        error_setg(errp, "inflight buffer subsection has not been loaded");
> +        return -EINVAL;
> +    }
> +
> +    return 0;
> +}
> +
> +const VMStateDescription vmstate_vhost_inflight_region = {
> +    .name = "vhost-inflight-region",
> +    .post_load_errp = vhost_inflight_region_post_load,
> +    .fields = (const VMStateField[]) {
> +        VMSTATE_UINT64(size, struct vhost_inflight),
> +        VMSTATE_UINT16(queue_size, struct vhost_inflight),
> +        VMSTATE_END_OF_LIST()
> +    },
> +    .subsections = (const VMStateDescription * const []) {
> +        &vmstate_vhost_inflight_region_buffer,
> +        NULL
> +    }
> +};
> +
>  const VMStateDescription vmstate_vhost_virtqueue = {
>      .name = "vhost-virtqueue",
>      .fields = (const VMStateField[]) {
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index 13ca2c319f..dd552de91f 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -596,6 +596,12 @@ extern const VMStateDescription vmstate_backend_transfer_vhost_inflight;
>                             vmstate_backend_transfer_vhost_inflight, \
>                             struct vhost_inflight)
>  
> +extern const VMStateDescription vmstate_vhost_inflight_region;
> +#define VMSTATE_VHOST_INFLIGHT_REGION(_field, _state) \
> +    VMSTATE_STRUCT_POINTER(_field, _state, \
> +                           vmstate_vhost_inflight_region, \
> +                           struct vhost_inflight)
> +
>  extern const VMStateDescription vmstate_vhost_dev;
>  #define VMSTATE_BACKEND_TRANSFER_VHOST(_field, _state) \
>      VMSTATE_STRUCT(_field, _state, 0, vmstate_vhost_dev, struct vhost_dev)
> -- 
> 2.34.1
> 

-- 
Peter Xu