Emulating device configuration / max_virtqueue_pairs in vhost-vdpa and vhost-user

[Eugenio Perez Martin]

Hi,

The current approach of offering an emulated CVQ to the guest and map
the commands to vhost-user is not scaling well:
* Some devices already offer it, so the transformation is redundant.
* There is no support for commands with variable length (RSS?)

We can solve both of them by offering it through vhost-user the same
way as vhost-vdpa do. With this approach qemu needs to track the
commands, for similar reasons as vhost-vdpa: qemu needs to track the
device status for live migration. vhost-user should use the same SVQ
code for this, so we avoid duplications.

One of the challenges here is to know what virtqueue to shadow /
isolate. The vhost-user device may not have the same queues as the
device frontend:
* The first depends on the actual vhost-user device, and qemu fetches
it with VHOST_USER_GET_QUEUE_NUM at the moment.
* The qemu device frontend's is set by netdev queues= cmdline parameter in qemu

For the device, the CVQ is the last one it offers, but for the guest
it is the last one offered in config space.

To create a new vhost-user command to decrease that maximum number of
queues may be an option. But we can do it without adding more
commands, remapping the CVQ index at virtqueue setup. I think it
should be doable using (struct vhost_dev).vq_index and maybe a few
adjustments here and there.

Thoughts?

Thanks!