Re: [dpdk-dev] [PATCH 0/4] Vhost: fix mq=on but VIRTIO_NET_F_MQ not negotiated

[Ladi Prosek <lprosek@redhat.com>]

On Mon, Dec 4, 2017 at 3:08 PM, Maxime Coquelin
<maxime.coquelin@redhat.com> wrote:
> Having QEMU started with mq=on but guest driver not negotiating
> VIRTIO_NET_F_MQ feature ends up in the vhost device to never
> start. Indeed, more queues are created in the vhost backend than
> configured.
>
> Guest drivers known to not advertise the VIRTIO_NET_F_MQ feature are
> iPXE and OVMF Virtio-net drivers.
>
> Queues are created because before starting the guest, QEMU sends
> VHOST_USER_SET_VRING_CALL requests for all queues declared in QEMU
> command line. Also, once Virtio features negotiated, QEMU sends
> VHOST_USER_SET_VRING_ENABLE requests to disable all but the first
> queue pair.
>
> This series fixes this by destroying all but first queue pair in
> the backend if VIRTIO_NET_F_MQ isn't negotiated.

The destroyed queue pairs will be re-created when an MQ-enabled driver
takes over later on, though, is that correct?

iPXE and OVMF virtio-net drivers will typically not be the last ones
driving the device. They will likely load an OS equipped with a driver
that negotiates the MQ feature. Thanks!

> First patches
> makes sure that VHOST_USER_SET_FEATURES request doesn't change
> Virtio features while the device is running, which should never
> happen as per the Virtio spec. This helps to make sure vitqueues
> aren't destroyed while being processed, but also protect from
> other illegal features changes (e.g. VIRTIO_NET_F_MRG_RXBUF).
>
> Maxime Coquelin (4):
>   vhost: prevent features to be changed while device is running
>   vhost: propagate VHOST_USER_SET_FEATURES handling error
>   vhost: extract virtqueue cleaning and freeing functions
>   vhost: destroy unused virtqueues when multiqueue not negotiated
>
>  lib/librte_vhost/vhost.c      | 22 ++++++++++++----------
>  lib/librte_vhost/vhost.h      |  3 +++
>  lib/librte_vhost/vhost_user.c | 40 ++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 53 insertions(+), 12 deletions(-)
>
> --
> 2.14.3
>