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

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

On Tue, Jan 31, 2023 at 08:11:06PM +0100, Eugenio Perez Martin wrote:
> On Tue, Jan 31, 2023 at 8:10 PM Eugenio Perez Martin
> <eperezma@redhat.com> 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?
> >
> > Thanks!
> 
> 
> (Starting a separated thread to vhost-vdpa related use case)
> 
> This could also work for vhost-vdpa if we ever decide to honor netdev
> queues argument. It is totally ignored now, as opposed to the rest of
> backends:
> * vhost-kernel, whose tap device has the requested number of queues.
> * vhost-user, that errors with ("you are asking more queues than
> supported") if the vhost-user parent device has less queues than
> requested (by vhost-user msg VHOST_USER_GET_QUEUE_NUM).
> 
> One of the reasons for this is that device configuration space is
> totally passthrough, with the values for mtu, rss conditions, etc.
> This is not ideal, as qemu cannot check src and destination
> equivalence and they can change under the feets of the guest in the
> event of a migration. External tools are needed for this, duplicating
> part of the effort.
> 
> Start intercepting config space accesses and offering an emulated one
> to the guest with this kind of adjustments is beneficial, as it makes
> vhost-vdpa more similar to the rest of backends, making the surprise
> on a change way lower.
> 
> Thoughts?
> 
> Thanks!

I agree here.

-- 
MST