Re: [RFC v2 06/13] vhost: delay set_vring_ready after DRIVER_OK

[Maxime Coquelin]

Hi Eugenio,

On 1/13/23 11:03, Eugenio Perez Martin wrote:
> On Fri, Jan 13, 2023 at 10:51 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
> > On Fri, Jan 13, 2023 at 09:19:00AM +0100, Eugenio Perez Martin wrote:
> > > On Fri, Jan 13, 2023 at 5:36 AM Jason Wang <jasowang@redhat.com> wrote:
> > > > On Fri, Jan 13, 2023 at 1:25 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> > > > > To restore the device at the destination of a live migration we send the
> > > > > commands through control virtqueue. For a device to read CVQ it must
> > > > > have received the DRIVER_OK status bit.
> > > >
> > > > This probably requires the support from the parent driver and requires
> > > > some changes or fixes in the parent driver.
> > > >
> > > > Some drivers did:
> > > >
> > > > parent_set_status():
> > > > if (DRIVER_OK)
> > > >      if (queue_enable)
> > > >          write queue_enable to the device
> > > >
> > > > Examples are IFCVF or even vp_vdpa at least. MLX5 seems to be fine.
> > >
> > > I don't get your point here. No device should start reading CVQ (or
> > > any other VQ) without having received DRIVER_OK.
> > >
> > > Some parent drivers do not support sending the queue enable command
> > > after DRIVER_OK, usually because they clean part of the state like the
> > > set by set_vring_base. Even vdpa_net_sim needs fixes here.
> > >
> > > But my understanding is that it should be supported so I consider it a
> > > bug. Especially after queue_reset patches. Is that what you mean?
> > >
> > > > > However this opens a window where the device could start receiving
> > > > > packets in rx queue 0 before it receives the RSS configuration. To avoid
> > > > > that, we will not send vring_enable until all configuration is used by
> > > > > the device.
> > > > >
> > > > > As a first step, run vhost_set_vring_ready for all vhost_net backend after
> > > > > all of them are started (with DRIVER_OK). This code should not affect
> > > > > vdpa.
> > > > >
> > > > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > > > ---
> > > > >   hw/net/vhost_net.c | 17 ++++++++++++-----
> > > > >   1 file changed, 12 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> > > > > index c4eecc6f36..3900599465 100644
> > > > > --- a/hw/net/vhost_net.c
> > > > > +++ b/hw/net/vhost_net.c
> > > > > @@ -399,6 +399,18 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
> > > > >           } else {
> > > > >               peer = qemu_get_peer(ncs, n->max_queue_pairs);
> > > > >           }
> > > > > +        r = vhost_net_start_one(get_vhost_net(peer), dev);
> > > > > +        if (r < 0) {
> > > > > +            goto err_start;
> > > > > +        }
> > > > > +    }
> > > > > +
> > > > > +    for (int j = 0; j < nvhosts; j++) {
> > > > > +        if (j < data_queue_pairs) {
> > > > > +            peer = qemu_get_peer(ncs, j);
> > > > > +        } else {
> > > > > +            peer = qemu_get_peer(ncs, n->max_queue_pairs);
> > > > > +        }
> > > >
> > > > I fail to understand why we need to change the vhost_net layer? This
> > > > is vhost-vDPA specific, so I wonder if we can limit the changes to e.g
> > > > vhost_vdpa_dev_start()?
> > >
> > > The vhost-net layer explicitly calls vhost_set_vring_enable before
> > > vhost_dev_start, and this is exactly the behavior we want to avoid.
> > > Even if we make changes to vhost_dev, this change is still needed.
> >
> > I'm working on something similar since I'd like to re-work the following
> > commit we merged just before 7.2 release:
> >       4daa5054c5 vhost: enable vrings in vhost_dev_start() for vhost-user
> >       devices
> >
> > vhost-net wasn't the only one who enabled vrings independently, but it
> > was easy enough for others devices to avoid it and enable them in
> > vhost_dev_start().
> >
> > Do you think can we avoid in some way this special behaviour of
> > vhost-net and enable the vrings in vhost_dev_start?
>
> Actually looking forward to it :). If that gets merged before this
> series, I think we could drop this patch.
>
> If I'm not wrong the enable/disable dance is used just by vhost-user
> at the moment.
>
> Maxime, could you give us some hints about the tests to use to check
> that changes do not introduce regressions in vhost-user?

You can use DPDK's testpmd [0] tool with Vhost PMD, e.g.:

#single queue pair
# dpdk-testpmd -l <CORE IDs> --no-pci 
--vdev=net_vhost0,iface=/tmp/vhost-user1 -- -i

#multiqueue
# dpdk-testpmd -l <CORE IDs> --no-pci 
--vdev=net_vhost0,iface=/tmp/vhost-user1,queues=4 -- -i --rxq=4 --txq=4

[0]: https://doc.dpdk.org/guides/testpmd_app_ug/index.html

Maxime

> Thanks!
>
> > Thanks,
> > Stefano
> >
> > > And we want to explicitly enable CVQ first, which "only" vhost_net
> > > knows which is. To perform that in vhost_vdpa_dev_start would require
> > > quirks, involving one or more of:
> > > * Ignore vq enable calls if the device is not the CVQ one. How to
> > > signal what is the CVQ? Can we trust it will be the last one for all
> > > kind of devices?
> > > * Enable queues that do not belong to the last vhost_dev from the enable call.
> > > * Enable the rest of the queues from the last enable in reverse order.
> > > * Intercalate the "net load" callback between enabling the last
> > > vhost_vdpa device and enabling the rest of devices.
> > > * Add an "enable priority" order?
> > >
> > > Thanks!
> > >
> > > > Thanks
> > > >
> > > > >           if (peer->vring_enable) {
> > > > >               /* restore vring enable state */
> > > > > @@ -408,11 +420,6 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
> > > > >                   goto err_start;
> > > > >               }
> > > > >           }
> > > > > -
> > > > > -        r = vhost_net_start_one(get_vhost_net(peer), dev);
> > > > > -        if (r < 0) {
> > > > > -            goto err_start;
> > > > > -        }
> > > > >       }
> > > > >
> > > > >       return 0;
> > > > > --
> > > > > 2.31.1