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Re: type=ioemu? (was: Re: NetBSD in an HVM instance can't see outside world)

On Thursday 04 September 2008 12:53:33 Johan Ihren wrote:
> Hi,
> On 3 Sep 2008, at 19:34, Manuel Bouyer wrote:
> >> vif = [ 'mac=00:16:3d:00:00:13, bridge=bridge0, type=ioemu' ]
> >
> > This should create a tap interface in the dom0, in addition to the
> > usual xvif. Please check that the tap interface is up, and was added to
> > bridge0
> Exactly what does the "type=ioemu" stuff mean? And why is the tap
> needed for networking? And what does it do in this context (i.e. I
> know that a tap is a virtual Ethernet in the DOM0, but at least for PV
> the xvif is all that's needed).

ioemu is a part from qemu used in Xen to emulate the io devices
for HVM guests. ioemu uses tap for the emulated ethernet device.

It is possible to use paravirtualized drivers in HVM guests for
(much) better io performance. xvif is used only by a PV ethernet
driver. Since Dom0 does not know if a HVM guest has PV drivers
or not, both tap and xvif are provided.

type=ioemu  explicitely specifies that ioemu has to be used.

> And, while on the subject of ioemu, I see things like:
> > disk = [ 'file:/xen/windows/winxp.img,ioemu:hda,w', 'phy:/dev/
> > cd0a,ioemu:hdb:cdrom,r' ]
> So how is that different from not using the "ioemu:" part in front of
> the VDEV (i.e. "hda" here)?

VDEV tells the Linux guest as which device the physical one should
appear in the guest. NetBSD guests don't use this.


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