Re: [Xen-users] How do unmodified guests work with Xen?

[<pjn@xxxxxxxxxxxx>]

"A port of Windows XP was developed for an earlier version of Xen, but is 
not available for release due to licence restrictions"
Presumably something close to the Windows standard driver model was used to 
make it work for interfacing to the underlying virtualised hardware. Is this 
support available. Clearly the modified version of XP itself would not be 
but why not the hardware/driver support if it exists?
Of course I could be entirely off-base here but it never hurts to ask :-)

..pjn
----- Original Message ----- 
From: "Mark Williamson" <mark.williamson@xxxxxxxxxxxx>
To: <xen-users@xxxxxxxxxxxxxxxxxxx>
Cc: "Ernst Bachmann" <e.bachmann@xxxxxxxx>; "Stephan AustermÃhle" 
<au@xxxxxxx>; <pjn@xxxxxxxxxxxx>
Sent: Wednesday, January 25, 2006 8:36 PM
Subject: Re: [Xen-users] How do unmodified guests work with Xen?


> > AFAIK it just works like it does in qemu, bochs, vmware, ...
> > The guest sees an (S)VGA and network device, accesses to that (virtual)
> > device are trapped and passed to an hardware emulator.
> Exactly.  In more detail:
>
> The Xen hypervisor itself remains a very small piece of software, and 
> doesn't
> contain device emulation code (except for a few performance-critical 
> pieces
> of platform hardware).
>
> The guest can mostly run directly on the (VT-enabled) CPU.  Various 
> privileged
> CPU operations need to emulated to make this work properly.  Hardware 
> devices
> can essentially be emulated entirely by trapping guest attempts to access 
> IO
> memory regions, or IO ports and feeding these into a "model" of how the
> device will respond to these accesses.  Currently the device models run as 
> a
> daemon process in dom0 (the actual device emulation code is derived from
> QEmu).  Whenever an attempt to do mmio or port io is trapped by Xen, it
> passes the details up to the device model daemon, which feeds these 
> details
> into the device emulation.  The device emulation simulates the current 
> device
> state and (when necessary) maps the memory of the virtual machine and
> transfers data into it.
>
> Using this process, we're able to completely emulate real hardware devices
> (network card, block, video, eventually sound too) for the benefit of
> unmodified guests, which still expect to see these features.
>
> The architecture is expected to change to a higher-performance 
> implementation
> during the evolution of Xen's full virtualisation support.
>
> Does that help everybody?
>
> Cheers,
> Mark
>
> --
> Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
> Mark: To answer a question with a question: What use is a skateboard?
> Dave: Skateboards have wheels.
> Mark: My wheel has a wheel!
>
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