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

[Eugenio Perez Martin]

On Thu, Feb 2, 2023 at 4:45 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2023/2/1 19:48, Eugenio Perez Martin 写道:
> > On Wed, Feb 1, 2023 at 12:20 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >> 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 ...
> >>
> > The number of queues may not be the same between cmdline and the device.
> >
> > If vhost-user device cmdline wants more queues than offered by the
> > device qemu will print an error. But the reverse (to offer the same
> > number of queues or less than the device have) is valid at this
> > moment.
> >
> > If we add cvq with this scheme, cvq index will not be the same between
> > the guest and the device.
> >
> > vhost-vdpa totally ignores the queues parameter, so we're losing the
> > opportunity to offer a consistent config space in the event of a
> > migration. I suggest we should act the same way as I'm proposing here
> > with vhost-user, so:
> > * QEMU can block the migration in the case the destination cannot
> > offer the same number of queues.
> > * The guest will not see a change of the config space under its feets.
>
>
> As we discussed in the past, it would be easier to fail the device
> initialization in this case.
>

But qemu does not know the source config space, so it cannot check if
it is equal. I think qemu cmdline combined with the migration protocol
is the security measure about this. It already checks for features,
the plan is to extend that check for config space.

Thanks!

> Thanks
>
>
> >
> > Now there are other fields in the config space for sure (mtu, rss
> > size, etc), but I think the most complex case is the number of queues
> > because cvq.
> >
> > Is that clearer?
> >
> > Thanks!
> >
>