Re: [Qemu-devel] Re: [PATCH-RFC 13/13] virtio-net: connect to vhost net backend

[Paul Brook]

> On Wed, Feb 24, 2010 at 03:14:25AM +0000, Paul Brook wrote:
> > > vnet_hdr is IMHO a really bad example to copy from.
> > >
> > > vnet_hdr leaks into the migration state via n->has_vnet_hdr.  What this
> > > means is that if you want to migrate from -net tap -net
> > > nic,model=virtio to -net user -net nic,model=virtio, it will fail.
> > >
> > > This is a hard problem to solve in qemu though because it would require
> > > that we implement software GSO which so far, no one has stepped up to
> > > do.
> >
> > Or make virtio-net pass this on to the guest, and have that deal with the
> > problem.
> 
> This is exacly what we do, via feature bits.

AFAIK we only have static feature bits. There aren't useful for anything that 
the user may change on the fly (or via migration).

If you don't have a fallback implementation then the guest must be able to 
cope with this feature disappearing without warning. If we do have a software 
fallback then the feature bit is just for backwards compatibility, and should 
be enabled unconditionally (on current machine types).

Paul