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RE: [Xen-devel] Unmodified Windows etc on XEN 3.0 with Intel/VT or AMD/P

To: "Mark Williamson" <mark.williamson@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, adam@xxxxxxxxxxx
Subject: RE: [Xen-devel] Unmodified Windows etc on XEN 3.0 with Intel/VT or AMD/Pacifica
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Mon, 30 Jan 2006 11:39:15 +0100
Cc: Thorolf Godawa <nospam@xxxxxxxxx>
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Thread-topic: [Xen-devel] Unmodified Windows etc on XEN 3.0 with Intel/VT or AMD/Pacifica

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Mark Williamson
> Sent: 30 January 2006 02:04
> To: xen-devel@xxxxxxxxxxxxxxxxxxx; adam@xxxxxxxxxxx
> Cc: Thorolf Godawa
> Subject: Re: [Xen-devel] Unmodified Windows etc on XEN 3.0 
> with Intel/VT or AMD/Pacifica
> > You write an xen configuration file that has a 'cdrom' entry which 
> > points to your cd device, or an iso image. Then you tell 
> xen to boot 
> > from the cdrom. Would look something like this:
> >
> > cdrom = "/path/to/isofile" (or /dev/hdc if that is your 
> cdrom device) 
> > boot  = "d"
> >
> > also you'd want:
> > vnc = 1
> Or you can enable a local X Window, if that suits your setup.
> > You would either use the vnc connection, but I wouldn't 
> suggest that 
> > as its very slow and for me has mouse problems.
> I'm not sure to what extent VNC is to blame, and to what 
> extent it's the display device model.  Using the normal X 
> Window might improve this a bit, but the current display 
> emulator will never win any speed awards.
> > >- If I use a Processor with x86-64-support, should I install the 
> > >x86-64-version of Linux or just the i386 if most xenU-domains just 
> > >will be x86? The advantage with x86-64 is that it uses 
> memory above 
> > >4GB better but is there a disadvantage with x86-only-guests?
> >
> > I don't really have enough knowledge to comment on this.
> I'm not entirely sure I understand the question...
> If you're running on a host with > 4Gig memory, you *either* 
> need to run a PAE dom0 on PAE Xen (not sure if you can use VT 
> on PAE Xen at the moment...) or run a 64-bit dom0 on 64-bit 
> Xen.  Dom0 and Xen have to be matched.
> Therefore, if you want to use > 4Gig memory in the whole 
> system *or* you want to run *any* 64-bit guests, you need a 
> 64-bit dom0 on a 64-bit Xen.  Does that answer your question?

I would say that if you have more than 4G (actually, over 3.5GB, as PCI
devices usually take up around 0.5GB) of memory, you should use Xen in
64-bit mode, and that in itself forces Dom0 to be x86-64 model as the
Xen and Dom0 need to be the same "bitness". 

As for DomU's, you probably want to run those with Paravirtualization if
possible - at this point in time the full virtualization isn't fully
optimized, and the para-virt solutin should run a bit faster (and
probably a few less bugs too...). 

If you haven't got more than 3.5GB of RAM, I would say that running
32-bit Xen & Dom0 would be the most efficient as the page-table-size
will be smaller than in a 64-bit system (which FORCES the use of PAE ->
64-bit pagetable entries -> double the size of the page-table). 

> HTH,
> Mark
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