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RE: [Xen-devel] DomU-DomU Event Channel

To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>, "Koripella Srinivas" <talkwithsrinivas@xxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] DomU-DomU Event Channel
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Thu, 3 May 2007 18:55:53 +0200
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Thread-topic: [Xen-devel] DomU-DomU Event Channel

> -----Original Message-----
> From: Liang Yang [mailto:multisyncfe991@xxxxxxxxxxx] 
> Sent: 03 May 2007 17:28
> To: Petersson, Mats; Koripella Srinivas; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] DomU-DomU Event Channel
> If you assign a DomU "driver-domain", does that cause unnecessary 
> communication overhead between your driver domain and domain0 
> as the native 
> driver modules still reside in the domain0? or you will put 
> your native 
> driver in domU together with your BE driver?

Short answer: No.

Long answer:
The definition of "driver domain" (at least as used here) is a domain
that has acces directly to the actual hardware, and thus can act as a
back-end for another domain with frontend drivers. There is normally no
communication between Dom0 and a driver-domain, certainly not in the
context of accessing the hardware assigned to the driver-domain, but
there can (usually is) some communication where the driver-domain uses
Dom0 to communicate to hardware that it itself doesn't own. 

For example, if we give a network card (but no other hardware devices)
to a DomU (name this "NetDomU"), and assign another DomU "UseDomU" to
use the network hardware in "NetDomU" via the normal BE/FE driver pair.
NetDomU can still not access the HARD-DISK, since it's not been given
access to any hard-disk controller. So if NetDomU is reading/writing to
a file for some reason, it communicates with Dom0 to do this. 

UseDomU will probably also use Dom0 to do disk-io, but the network
traffic (at least on some interface) doesn't pass Dom0, only NetDomU. 

I hope this clarfies things. 

> Liang
> ----- Original Message ----- 
> From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
> To: "Koripella Srinivas" <talkwithsrinivas@xxxxxxxxxxx>; 
> <xen-devel@xxxxxxxxxxxxxxxxxxx>
> Sent: Thursday, May 03, 2007 9:01 AM
> Subject: RE: [Xen-devel] DomU-DomU Event Channel
> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> > Koripella Srinivas
> > Sent: 03 May 2007 16:55
> > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > Subject: [Xen-devel] DomU-DomU Event Channel
> >
> > Hi,
> > I want to move the qemu emulation code running in domain0 and
> > move it to another domain for a particular device for a HVM
> > guest. As part of that i want to know if it is possible to
> > set up an event channel between 2 domU's.
> I would have thought so, as you can assign a DomU 
> "driver-domain" to be
> the backend to another DomU - I take it that this would be 
> roughly what
> you're trying to do as well, except for a HVM-domain.
> [This is pure speculation, based on my understanding of how 
> domains talk
> to each other combined with the functinality described by "xm
> block/net-attach"].
> --
> Mats
> >
> > thanks
> >
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