Re: [PATCH v0 0/2] virtio-blk and vhost-user-blk cross-device migration

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

On Mon, Oct 04, 2021 at 06:07:29PM +0300, Denis Plotnikov wrote:
> It might be useful for the cases when a slow block layer should be replaced
> with a more performant one on running VM without stopping, i.e. with very low
> downtime comparable with the one on migration.
> 
> It's possible to achive that for two reasons:
> 
> 1.The VMStates of "virtio-blk" and "vhost-user-blk" are almost the same.
>   They consist of the identical VMSTATE_VIRTIO_DEVICE and differs from
>   each other in the values of migration service fields only.
> 2.The device driver used in the guest is the same: virtio-blk
> 
> In the series cross-migration is achieved by adding a new type.
> The new type uses virtio-blk VMState instead of vhost-user-blk specific
> VMstate, also it implements migration save/load callbacks to be compatible
> with migration stream produced by "virtio-blk" device.
> 
> Adding the new type instead of modifying the existing one is convenent.
> It ease to differ the new virtio-blk-compatible vhost-user-blk
> device from the existing non-compatible one using qemu machinery without any
> other modifiactions. That gives all the variety of qemu device related
> constraints out of box.

Hmm I'm not sure I understand. What is the advantage for the user?
What if vhost-user-blk became an alias for vhost-user-virtio-blk?
We could add some hacks to make it compatible for old machine types.

> 0001: adds new type "vhost-user-virtio-blk"
> 0002: add new type "vhost-user-virtio-blk-pci"
> 
> Denis Plotnikov (2):
>   vhost-user-blk: add a new vhost-user-virtio-blk type
>   vhost-user-blk-pci: add new pci device type to support
>     vhost-user-virtio-blk
> 
>  hw/block/vhost-user-blk.c          | 63 ++++++++++++++++++++++++++++++
>  hw/virtio/vhost-user-blk-pci.c     | 43 ++++++++++++++++++--
>  include/hw/virtio/vhost-user-blk.h |  2 +
>  3 files changed, 105 insertions(+), 3 deletions(-)
> 
> -- 
> 2.25.1