Re: [Qemu-devel] [RFC] vhost-user: don't ignore CTRL_VLAN feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 11, 2019 at 02:51:37PM +0800, Tiwei Bie wrote:
> The VIRTIO_NET_F_CTRL_VLAN feature requires the support of
> vhost-user backend. But it will be advertised to guest driver
> as long as it's enabled by users in QEMU, while it's not
> supported by vhost-user backend. This patch fixes this issue.

Fixes by making guest refuse to send vlan tags?
I agree it seems cleaner, but which guests does this actually help?

> Fixes: 72018d1e1917 ("vhost-user: ignore qemu-only features")
> Cc: qemu-stable@nongnu.org
> 
> Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>

A change like that will break migration compatibility, will it not?
Maybe we need to tie it to a machine version somehow...



> ---
> It's not clear in the spec that, whether vlan filtering is
> also best-effort:
> https://github.com/oasis-tcs/virtio-spec/blob/37057052e7/content.tex#L3372

So what breaks if we declare it best effort for now?
And does it really help if we report that vlan filtering
is not supported to guests?



>  hw/net/vhost_net.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index a6b719035c..1444fc9230 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -75,6 +75,8 @@ static const int user_feature_bits[] = {
>      VIRTIO_NET_F_MTU,
>      VIRTIO_F_IOMMU_PLATFORM,
>  
> +    VIRTIO_NET_F_CTRL_VLAN,
> +
>      /* This bit implies RARP isn't sent by QEMU out of band */
>      VIRTIO_NET_F_GUEST_ANNOUNCE,
>  
> -- 
> 2.17.1