Re: [dpdk-dev] [PATCH] net/virtio: fix virtio-user init when using existing tap

[David Marchand <david.marchand@redhat.com>]

On Thu, Sep 23, 2021 at 5:39 PM Olivier Matz <olivier.matz@6wind.com> wrote:
>
> Hi David,
>
> On Fri, Sep 17, 2021 at 11:33:44AM +0200, David Marchand wrote:
> > When attaching to an existing mono queue tap, the virtio-user was not
> > reporting that the virtio device was not properly initialised which
> > prevented from starting the port later.
> >
> > $ ip tuntap add test mode tap
> > $ dpdk-testpmd --vdev \
> >   net_virtio_user0,iface=test,path=/dev/vhost-net,queues=2 -- -i
> >
> > ...
> > virtio_user_dev_init_mac(): (/dev/vhost-net) No valid MAC in devargs or
> > device, use random
> > vhost_kernel_open_tap(): TUNSETIFF failed: Invalid argument
> > vhost_kernel_enable_queue_pair(): fail to open tap for vhost kernel
> > virtio_user_start_device(): (/dev/vhost-net) Failed to start device
> > ...
> > Configuring Port 0 (socket 0)
> > vhost_kernel_open_tap(): TUNSETIFF failed: Invalid argument
> > vhost_kernel_enable_queue_pair(): fail to open tap for vhost kernel
> > virtio_set_multiple_queues(): Multiqueue configured but send command
> > failed, this is too late now...
> > Fail to start port 0: Invalid argument
> > Please stop the ports first
> > Done
> >
> > The virtio-user with vhost-kernel backend was going through a lot
> > of complications to initialise tap fds only when using them.
> >
> > For each qp enabled for the first time, a tapfd was created via
> > TUNSETIFF with unneeded additional steps (see below) and then mapped to
> > the right qp in the vhost-net backend.
> > Unneeded steps (as long as it has been done once for the port):
> > - tap features were queried while this is a constant on a running
> >   system,
> > - the device name in DPDK was updated,
> > - the mac address of the tap was set,
> >
> > On subsequent qps state change, the vhost-net backend fd mapping was
> > updated and the associated queue/tapfd were disabled/enabled via
> > TUNSETQUEUE.
> >
> > Now, this patch simplifies the whole logic by keeping all tapfds opened
> > and in enabled state (from the tap point of view) at all time.
> >
> > Unused ioctl defines are removed.
> >
> > Tap features are validated earlier to fail initialisation asap.
> > Tap name discovery and mac address configuration are moved when
> > configuring qp 0.
> >
> > To support attaching to mono queue tap, the virtio-user driver now tries
> > to attach in multi queue first, then fallbacks to mono queue.
> >
> > Finally (but this is more for consistency), VIRTIO_NET_F_MQ feature is
> > exposed only if the underlying tap supports multi queue.
> >
> > Signed-off-by: David Marchand <david.marchand@redhat.com>
>
> It also solves our use-case, which is a bit different:
>
> - our application creates a tap with the IFF_MULTI_QUEUE
> - we bind this tap to a virtio-user pmd
>
> Previously, it was not working when passing queues=1 to the pmd, the
> TUNSETIFF ioctl() in the pmd was failing because the IFF_MULTI_QUEUE
> flag was not passed.
>
> With your patch, it works fine, thanks.

Cool.


>
> (...)
>
> > @@ -414,18 +425,40 @@ vhost_kernel_setup(struct virtio_user_dev *dev)
> >                       PMD_DRV_LOG(ERR, "fail to open %s, %s", dev->path, strerror(errno));
> >                       goto err_tapfds;
> >               }
> > -
> >               data->vhostfds[i] = vhostfd;
> >       }
> >
> > +     ifname = dev->ifname != NULL ? dev->ifname : "tap%d";
> > +     if (tap_features & IFF_MULTI_QUEUE)
> > +             data->tapfds[0] = tap_open(ifname, true);
> > +     if (data->tapfds[0] < 0)
> > +             data->tapfds[0] = tap_open(ifname, false);
>
> As discussed off-list, since tap_open() is called twice, it can in some
> conditions log failures twice. And since tun/tap multi queue feature is
> available from 3.8 kernel, we may not want to bother with that.

Let me relook at this for v2.


Thanks for the test!


-- 
David Marchand