[libvirt] Re: Supporting vhost-net and macvtap in libvirt for QEMU

Michael S. Tsirkin mst at redhat.com
Thu Dec 17 15:52:03 UTC 2009

On Wed, Dec 16, 2009 at 07:48:08PM -0600, Anthony Liguori wrote:
> Current, libvirt invokes qemu with -net tap,fd=X where X is an already  
> open fd to a tun/tap device.  I suspect that after we merge vhost-net,  
> libvirt could support vhost-net in this mode by just doing -net  
> vhost,fd=X.  I think the only real question for libvirt is whether to  
> provide a user visible switch to use vhost or to just always use vhost  
> when it's available and it makes sense.  Personally, I think the later  
> makes sense.

I was currently trying to implement
-net tap,fd=X,vhost
since I thought this is what you suggested originally.

> The more interesting invocation of vhost-net though is one where the  
> vhost-net device backs directly to a physical network card.  In this  
> mode, vhost should get considerably better performance than the current  
> implementation.  I don't know the syntax yet, but I think it's  
> reasonable to assume that it will look something like -net tap,dev=eth0.  
>  The effect will be that eth0 is dedicated to the guest.


> modeled in libvirt?  Michael, could you share your current thinking for  
> -net syntax?

I don't really care much.
>From networking POV, these two are similar cases: vepa
where an external bridge loops packets back at host,
and SRIOV where this is done internally by the card,
but in both cases it is external to host.
For this reason, I thought that we might want to call it
like this: -net external,eth0

> -- 
> Regards,
> Anthony Liguori

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