Re: [PATCH 0/2] Selective queue enabling

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed, 7 Jun 2023 07:35:58 +0200, Eugenio Perez Martin <eperezma@redhat.com> wrote:
> On Tue, Jun 6, 2023 at 9:10âPM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Jun 06, 2023 at 07:55:09PM +0200, Eugenio PÃrez wrote:
> > > This series allows the driver to start the device (as set DRIVER_OK) with only
> > > some queues enabled, and then enable another queues later.
> > >
> > > This is the current way to migrate net device state through control
> > > virtqueue, in a software assisted framework with vDPA:
> > > * First, only net CVQ is enabled at DRIVER_OK
> > > * All the control commands (mac address, mq, etc) needed for the device
> > > to behave the same as the source of migration are sent
> > > * Finally all the dataplane queues are enabled.
> >
> > In my opinion, this is somewhat problematic. Specifically, currently
> > devices tend to deduce how many queues are needed by looking
> > at the state at DRIVER_OK time.
> >
> > Question: what is wrong with enabling queues initially and then
> > doing a reset right after DRIVER_OK? You can even allocate
> > memory for just one queue (zeroing it out).
> >
> > Granted this looks kind of ugly but side-steps this problem with
> > no need for spec changes.
> >
>
> The problem is that the rx queues can start receiving, as the guest
> already has buffers there.

Can we reset the vq before filling buffers to it?

> Apart from that, the back and forth
> introduces latencies.
>
> Maybe a better angle is to start all the queues as if they're reset,
> write 1 just to CVQ, configure the device, and then write 1 to all
> dataplane vqs?

write to what?

Thanks.

>
> Thanks!
>
> > > Eugenio PÃrez (2):
> > >   virtio: introduce selective queue enabling
> > >   virtio: pci support virtqueue selective enabling
> > >
> > >  content.tex       | 15 +++++++++++++--
> > >  transport-pci.tex |  4 ++++
> > >  2 files changed, 17 insertions(+), 2 deletions(-)
> > >
> > > --
> > > 2.31.1
> > >
> >
>