Re: [Xen-devel] Re: Pre-virtualization, was Re: linux/arch/xen/i386 or linux/arch/i386/xen

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Ronald G. Minnich wrote:

> On Sat, 21 May 2005, aq wrote:
>  
>
> > Joshua, as I understand, this project would be a competitor of Xen?
> >    
> http://l4hq.org/cvsweb/cvsweb/~checkout~/afterburner/afterburn-wedge/doc/userman.pdf?content-type=application/pdf
>
> for the manual.
>
> You still have to modify the kernel, it seems. 
>  
Strictly speaking you should be able to achieve "full virtualization" 
with toolchain modifications.  Think of the comparision to VMX.  VMX 
provides exits to call into the hypervisor when certain instructions are 
executed that are not virtualization friendly.  With pre-virtualization, 
you have the compiler pad virtualization unfriendly instructions with 
nop's and then at load time, replace all of those unfriendly sequences 
with calls to the hypervisor.  This way, the same kernel can run on bare 
metal or within a hypervisor.
Of course, just as is likely with VMX, you're probably gonna want to do 
some hand-tuning of Linux for performance reasons.  It seems like 
afterburner incorporates these types of optimizations.
I'd really like to see a "pure" form of pre-virtualization that required 
no modifications at all to the underlying source tree.  Besides being 
interesting from an academic standpoint I think it would be highly 
useful for support legacy Open Source operating systems.
I'm very excited about this technology.  I imagine that you can get all 
the benefits of binary-rewriting with less complexity and better 
performance (with the only limitation being that you have the source 
code for the OS which is fine by me).
Regards,
Anthony Liguori

> Are there fewer mods? What is the advantage of this over Xen? I think I'm 
> missing a key point here. 
>
> ron
>
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