Re: [Qemu-devel] [PATCH] vhost_net: do not expose MTU feature bit to kernel backend

[Maxime Coquelin]

On 05/22/2017 07:24 PM, Michael S. Tsirkin wrote:
> On Sat, May 20, 2017 at 10:06:58AM +0200, Maxime Coquelin wrote:
> > This patch removes MTU from the list of features supported by
> > the kernel backend, so that vhost kernel backend does not have
> > to advertise it to be negotiated.
> >
> > Cc: Aaron Conole <address@hidden>
> > Suggested-by: Michael S. Tsirkin <address@hidden>
> > Signed-off-by: Maxime Coquelin <address@hidden>
>
> This will break cross-version migration when the kernel
> does not advertise it though. I'm afraid we'll need
> a compat flag.

Ok, you are right.
The difficulty with a compat flag is that vhost-net is not a driver,
so we cannot add it a new property to make it kernel-backend only.

For consistency, maybe we can add a new internal property to virtio-net
driver, to bypass backend negotiation for this feature for all backends?

I will propose a new version shortly.

Thanks,
Maxime
> > ---
> >   hw/net/vhost_net.c | 1 -
> >   1 file changed, 1 deletion(-)
> >
> > diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> > index 22874a9..bdd8633 100644
> > --- a/hw/net/vhost_net.c
> > +++ b/hw/net/vhost_net.c
> > @@ -51,7 +51,6 @@ static const int kernel_feature_bits[] = {
> >       VIRTIO_RING_F_EVENT_IDX,
> >       VIRTIO_NET_F_MRG_RXBUF,
> >       VIRTIO_F_VERSION_1,
> > -    VIRTIO_NET_F_MTU,
> >       VIRTIO_F_IOMMU_PLATFORM,
> >       VHOST_INVALID_FEATURE_BIT
> >   };
> > --
> > 2.9.3