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.
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