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

[Michael S. Tsirkin]

On Wed, Feb 01, 2023 at 12:14:18PM +0100, Maxime Coquelin wrote:
> Thanks Eugenio for working on this.
> 
> On 1/31/23 20:10, Eugenio Perez Martin wrote:
> > 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?
> 
> I am fine with both proposals.
> I think index remapping will require a bit more rework in the DPDK
> Vhost-user library, but nothing insurmountable.
> 
> I am currently working on a PoC adding support for VDUSE in the DPDK
> Vhost library, and recently added control queue support. We can reuse it
> if we want to prototype your proposal.
> 
> Maxime
> 
> > Thanks!
> > 


technically backend knows how many vqs are there, last one is cvq...
not sure we need full blown remapping ...

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MST