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

[Michael S. Tsirkin]

On Wed, Feb 24, 2010 at 12:26:53PM +0000, Paul Brook wrote:
> > > 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).
> >
> > Software fallback might turn out to be slower than disabling the feature
> > in the guest. For example, and extra pass over packet might cause extra CPU
> > cache thrashing. In that case, it's not obvious whether enabling it
> > unconditionally will turn out to be a good idea. But we'll have to have
> > that code to be able to tell.
> 
> IMO once you accept that these things can change, consistency is more 
> important than trying to guess what the "best" option may be.

Yes, SW fallback might be nice to have. What's important is likely to
depend on specific user.

> Starting qemu on machine A and migrating to machine B should give the same 
> guest environment as starting on machine B.
> 
> Paul

So currently, the way we try to ensure this is by checking feature bits
against the list supported by backend, and failing migration if there's
a discrepancy.


-- 
MST