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

[Michael S. Tsirkin]

On Wed, Feb 01, 2023 at 11:29:35AM +0800, Jason Wang wrote:
> On Wed, Feb 1, 2023 at 3:11 AM Eugenio Perez Martin <eperezma@redhat.com> 
> 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.
> 
> This looks not the responsibility of qemu but the upper layer (to
> provision the same config/features in src/dst).
> 
> > 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.
> 
> This probably needs more thought, since vDPA already provides a kind
> of emulation in the kernel. My understanding is that it would be
> sufficient to add checks to make sure the config that guests see is
> consistent with what host provisioned?
> 
> Thanks

I feel ability to adjust and emulate one device on top of another
one is a strong suit of virtio (as opposed to e.g. vfio which
always wants exactly the same device). Let's not throw this baby
out with the bathwater.

> >
> > Thoughts?
> >
> > Thanks!
> >