Re: [Qemu-devel] Re: [PATCH-updated] qemu/net: add raw backend

[Anthony Liguori]

Jamie Lokier wrote:
> Anthony Liguori wrote:
>   
> > Or Gerlitz wrote:
> >     
> > > Add raw network backend option which uses a packet socket to provide
> > > raw networking access. Once the socket is opened it's bound to a
> > > provided host interface, such that packets received on the interface
> > > are delivered to the VM and packets sent by the VM are sent to the
> > > interface.
> > >
> > > This is functionally similar to the existing pcap network
> > > backend, with the same advantages and problems.
> > > Differences from pcap:
> > > - can get an open socket from the monitor,
> > >  which allows running without NET_ADMIN priviledges
> > > - support iovec sends with writev, saving one data copy
> > > - one less dependency on an external library
> > > - we have access to the underlying file descriptor
> > >  which makes it possible to connect to vhost net
> > > - don't support polling all interfaces, always bind to a specific one
> > >  
> > >       
> > Networking is probably the area in qemu that users most frequently 
> > stumble with.  The most common problems are:
> >
> > 1) slirp does not behave how they think it should (icmp doesn't work, 
> > guest isn't accessable from host)
> > 2) it's difficult to figure out which backend behaves the way they want 
> > (socket vs. vde vs. tap)
> > 3) when they figure out they need tap, tap is difficult to setup 
> >     
>
> Worse, tap is impossible to setup properly with things like
> network-manager.
>   

This is being fixed.

> I suspect user expectations are quite commonly:
>
>    - guest<->host networking works
>    - guest<->host's network works, directly or through host NAT
>    - guest IP address is either private (inside the host)
>      or on the same network as the host, according to some switch.
>
> Imho, there is only one right place to fix this, and it's by adding a
> feature to the host.  Either modifying host packet socket, or
> modifying the tap+bridge combination.
>
> Neither tap nor pcap/raw works particularly well except in static IP
> configurations, and qemu cannot realistically work around the
> host configuration difficulties.
>   

The fact that network manager does work well with bridged interfaces is 
a network manager bug.  It's getting fixed so in the near future, tap 
will satisfy all of these requirements.

> It'd be great if vhost_net doesn't have the configuration problems of
> tap or pcap/raw.  If it does have the same problems, it's a natural
> place to fix them.  I haven't looked at vhost_net yet.
>
> -- Jamie
>   

Regards,

Anthony Liguori