RE: [Xen-devel] HVM guest sees two NICs even though just one vif configured

["Fischer, Anna" <anna.fischer@xxxxxx>]

> Subject: RE: [Xen-devel] HVM guest sees two NICs even though just one
> vif configured
> 
> On Thu, 6 Aug 2009, Fischer, Anna wrote:
> > > Subject: RE: [Xen-devel] HVM guest sees two NICs even though just
> one
> > > vif configured
> > >
> > > On Thu, 6 Aug 2009, Fischer, Anna wrote:
> > > > > Subject: Re: [Xen-devel] HVM guest sees two NICs even though
> just
> > > one
> > > > > vif configured
> > > > >
> > > > > On 06/08/2009 10:32, "Fischer, Anna" <anna.fischer@xxxxxx>
> wrote:
> > > > >
> > > > > > For me this does not sound right. If I configure my guest
> with
> > > only a
> > > > > single
> > > > > > NIC in the configuration file, then there should not be any
> way
> > > for
> > > > > the guest
> > > > > > to set up and use two NICs at the same time.
> > > > > >
> > > > > > Is this a known issue? How is this supposed to be handled?
> > > > >
> > > > > I think it's deliberate, to allow install of an OS with
> emulated
> > > > > driver,
> > > > > followed by switchover to PV drivers, wth no need for config
> > > changes.
> > > >
> > > > I assumed that was the reason behind this. However, a switchover
> > > would need to guarantee that a guest cannot use two NICs at the
> same
> > > time, doesn't it? Because otherwise the guest can send out of two
> > > separate devices at the same time and it could cause unexpected
> > > behaviour unless it is explicitly prevented at the backends in
> Dom0,
> > > e.g. through filtering or anti-spoofing I guess.
> > > >
> > >
> > > I more recent qemu allows the pv drivers in the guest to disconnect
> the
> > > emulated devices to avoid these problems.
> >
> > I am not too confident with having to rely on the guest to make such
> a disconnect happen. Is there no way to do this in Xen/Dom0?
> >
> 
> The idea is that the pv drivers should load only after having
> disabled the emulated devices.
> After all if the pv drivers are badly written you can end up with a
> corrupted disk anyway.

A corrupted PV NIC cannot that easily send packet through. Having the guest 
transmitting through two interfaces while only one is "officially" configured 
on the system is a different scenario though, I would say. I am not saying it 
is not an (easily) solvable issue. But it does require some mechanisms to be 
set up in Dom0 explicitly. The running instantiation of the guest (having two 
NICs) simply differs from what is specified on the system (my config file says 
the guest has one NIC). That inconsistency might cause problems.

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