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Re: [Xen-devel] [PATCH] create proper net device for hvm guests

To: Jim Fehlig <jfehlig@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] create proper net device for hvm guests
From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Date: Sat, 5 Apr 2008 01:30:11 +0100
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On Fri, Apr 04, 2008 at 05:25:57PM -0600, Jim Fehlig wrote:
> I've noticed that hvm guests using pv network device also get an
> emulated network device.  Conversely, hvm guests using emulated network
> device also get pv network device.  I'm surprised that this hasn't been
> fixed already, which makes me think there is some unforeseen issue that
> results from not providing both.  I guess one drawback is that the guest
> config file must be edited to remove 'model=foo, type=ioemu' when using
> pv network device  - but it seems the guest config file should be the
> place where these types of settings are explicitly stated.
> I've attached a patch the creates either the emulated or pv network
> device (but not both) depending on setting in guest configuation, i.e.
> 'type=ioemu'.  Please apply if my reasoning is sane.  If not, please
> clarify reason for presenting both :-).

The guest administrator does not have control over the Dom0, and thus has
no ability to change the config. Since installing Xen PV drivers is a post
install task, you can't create the guest with pv-only mode straightaway,
and nor can the guest admin change the config. Thus the presenting both the
devices lets the guest admin choose between the two at will. They can
easily blacklist the '8139too' driver in modprobe.conf if they only want
to use the pv driver and prevent the 2nd nic from being mistakenly used.

IIRC, the logic is supposed to currently be:

  - type=ioemu  - only present emulated rtl8139 (or as per model=)
  - type=netfront - only present the pv driver
  - no type= setting - present both pv & rtl8139 nic

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