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!