[v3,5/5] vhost-user-fs: Implement internal migration

A virtio-fs device's VM state consists of:
- the virtio device (vring) state (VMSTATE_VIRTIO_DEVICE)
- the back-end's (virtiofsd's) internal state

We get/set the latter via the new vhost operations to transfer migratory
state.  It is its own dedicated subsection, so that for external
migration, it can be disabled.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 hw/virtio/vhost-user-fs.c | 101 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 100 insertions(+), 1 deletion(-)

On Fri, Sep 15, 2023 at 12:25:30PM +0200, Hanna Czenczek wrote:
> A virtio-fs device's VM state consists of:
> - the virtio device (vring) state (VMSTATE_VIRTIO_DEVICE)
> - the back-end's (virtiofsd's) internal state
> 
> We get/set the latter via the new vhost operations to transfer migratory
> state.  It is its own dedicated subsection, so that for external
> migration, it can be disabled.
> 
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  hw/virtio/vhost-user-fs.c | 101 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 100 insertions(+), 1 deletion(-)

CCing Anton so he is aware that internal migration is being added.

> 
> diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
> index 49d699ffc2..eb91723855 100644
> --- a/hw/virtio/vhost-user-fs.c
> +++ b/hw/virtio/vhost-user-fs.c
> @@ -298,9 +298,108 @@ static struct vhost_dev *vuf_get_vhost(VirtIODevice *vdev)
>      return &fs->vhost_dev;
>  }
>  
> +/**
> + * Fetch the internal state from virtiofsd and save it to `f`.
> + */
> +static int vuf_save_state(QEMUFile *f, void *pv, size_t size,
> +                          const VMStateField *field, JSONWriter *vmdesc)
> +{
> +    VirtIODevice *vdev = pv;
> +    VHostUserFS *fs = VHOST_USER_FS(vdev);
> +    Error *local_error = NULL;
> +    int ret;
> +
> +    ret = vhost_save_backend_state(&fs->vhost_dev, f, &local_error);
> +    if (ret < 0) {
> +        error_reportf_err(local_error,
> +                          "Error saving back-end state of %s device %s "
> +                          "(tag: \"%s\"): ",
> +                          vdev->name, vdev->parent_obj.canonical_path,
> +                          fs->conf.tag ?: "<none>");
> +        return ret;
> +    }
> +
> +    return 0;
> +}
> +
> +/**
> + * Load virtiofsd's internal state from `f` and send it over to virtiofsd.
> + */
> +static int vuf_load_state(QEMUFile *f, void *pv, size_t size,
> +                          const VMStateField *field)
> +{
> +    VirtIODevice *vdev = pv;
> +    VHostUserFS *fs = VHOST_USER_FS(vdev);
> +    Error *local_error = NULL;
> +    int ret;
> +
> +    ret = vhost_load_backend_state(&fs->vhost_dev, f, &local_error);
> +    if (ret < 0) {
> +        error_reportf_err(local_error,
> +                          "Error loading back-end state of %s device %s "
> +                          "(tag: \"%s\"): ",
> +                          vdev->name, vdev->parent_obj.canonical_path,
> +                          fs->conf.tag ?: "<none>");
> +        return ret;
> +    }
> +
> +    return 0;
> +}
> +
> +static bool vuf_is_internal_migration(void *opaque)
> +{
> +    /* TODO: Return false when an external migration is requested */
> +    return true;
> +}
> +
> +static int vuf_check_migration_support(void *opaque)
> +{
> +    VirtIODevice *vdev = opaque;
> +    VHostUserFS *fs = VHOST_USER_FS(vdev);
> +
> +    if (!vhost_supports_device_state(&fs->vhost_dev)) {
> +        error_report("Back-end of %s device %s (tag: \"%s\") does not support "
> +                     "migration through qemu",
> +                     vdev->name, vdev->parent_obj.canonical_path,
> +                     fs->conf.tag ?: "<none>");
> +        return -ENOTSUP;
> +    }
> +
> +    return 0;
> +}
> +
> +static const VMStateDescription vuf_backend_vmstate;
> +
>  static const VMStateDescription vuf_vmstate = {
>      .name = "vhost-user-fs",
> -    .unmigratable = 1,
> +    .version_id = 0,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_VIRTIO_DEVICE,
> +        VMSTATE_END_OF_LIST()
> +    },
> +    .subsections = (const VMStateDescription * []) {
> +        &vuf_backend_vmstate,
> +        NULL,
> +    }
> +};
> +
> +static const VMStateDescription vuf_backend_vmstate = {
> +    .name = "vhost-user-fs-backend",
> +    .version_id = 0,
> +    .needed = vuf_is_internal_migration,
> +    .pre_load = vuf_check_migration_support,
> +    .pre_save = vuf_check_migration_support,
> +    .fields = (VMStateField[]) {
> +        {
> +            .name = "back-end",
> +            .info = &(const VMStateInfo) {
> +                .name = "virtio-fs back-end state",
> +                .get = vuf_load_state,
> +                .put = vuf_save_state,
> +            },
> +        },
> +        VMSTATE_END_OF_LIST()
> +    },
>  };
>  
>  static Property vuf_properties[] = {
> -- 
> 2.41.0
>