RE: [Xen-devel] Xen on top of xen

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: M.A. Williamson [mailto:maw48@xxxxxxxxxxxxxxxx] On 
> Behalf Of Mark Williamson
> Sent: 10 May 2007 10:45
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Cc: Petersson, Mats; Koripella Srinivas
> Subject: Re: [Xen-devel] Xen on top of xen
> 
> > > Can u throw more light on  the non-nestability ??
> >
> > Well, to put it simply, once you set up the SVM/VMX 
> feature, it doesn't
> > allow another such setup on top of it. It would be 
> technically possible
> > to support this in a processor designed to do so, but at the moment,
> > neither of AMD's or Intel's CPU's support this.
> 
> It could be trapped and emulated in Xen though, right?  The 
> AMD manual was 
> pretty clear on all the SVM instructions being trappable.  
> The VT-x manual I 
> had access to when I looked into this was a bit vague in this area.

Yes, at least for SVM you could trap and emulate. 
> 
> e.g. a scheme whereby the "outermost" Xen trapped SVM 
> instructions issued by 
> the current nested guest and built up a shadow copy of 
> HVM-related data 
> structures, then ran the "inner" HVM guest using these?  
> Future additions to 
> SVM implementations (such as nested paging) would reduce the 
> frequency with 
> which you have to bounce back into the guests.

I'm not sure if it's worth the effort tho'. It would be relatively slow
(because you'd first get the penalty of the VMExit/VMRun pair, and then
the additional code to emulate the VMExit/VMRun set.

> 
> > It is clearly easier to 
> > design a processor that only supports one level of "nesting" of
> > virtualization[1], and it is also debatable what the REAL benefit of
> > running nested virtual machines would be - I'm sure if it was a huge
> > benefit compared to the effort of implementing it[2], at 
> least one of
> > the two producers of virtualizable x86 processors would 
> have added such
> > fearures...
> 
> Being able to run HVM Xen in HVM Xen could be useful for 
> testing.  Also, from 
> a management perspective it would make it possible to 
> subdivide the machine 
> between multiple users who each wanted to run a few virtual 
> machines - just 
> give them each a Xen in an HVM domain.  Further levels of 
> nesting would be 
> overkill, and probably give horrible performance.

Of those, the management one is the more applicable to our engineering
efforts (i.e. the one that will "sell more processors"), but I'm pretty
sure that the effort is much greater than the gain. 

Do you (or anyone else) know if the PPC version supports nested
virtualization?

--
Mats
> 
> Cheers,
> Mark
> 
> > [1] Keeping one set of data for where to return to when the VMExit
> > happens, for example, is easier than having a "stack" of such.
> >
> > [2] On top of the extra effort to design and test such 
> feature, there is
> > also a potential performance penalty for nesting VM's, 
> since there is
> > more logic involved. Depending on where that extra logic 
> ends up, it may
> > be necessary to add a further clock-cycle to some operation.
> >
> > --
> > Mats
> >
> > > ----- Original Message ----
> > > From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
> > > To: pradeep singh rautela <rautelap@xxxxxxxxx>; Koripella
> > > Srinivas <talkwithsrinivas@xxxxxxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx;
> > > xen-devel-request@xxxxxxxxxxxxxxxxxxx
> > > Sent: Wednesday, 9 May, 2007 4:33:57 PM
> > > Subject: RE: [Xen-devel] Xen on top of xen
> > >
> > > > -----Original Message-----
> > > > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> > > > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> > > > pradeep singh rautela
> > > > Sent: 09 May 2007 10:11
> > > > To: Koripella Srinivas
> > > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx;
> > > > xen-devel-request@xxxxxxxxxxxxxxxxxxx
> > > > Subject: Re: [Xen-devel] Xen on top of xen
> > > >
> > > >
> > > >
> > > > On 5/9/07, pradeep singh rautela <rautelap@xxxxxxxxx> wrote:
> > > >
> > > >
> > > >
> > > >     On 5/9/07, Koripella Srinivas <
> > > > talkwithsrinivas@xxxxxxxxxxx
> > > > <mailto:talkwithsrinivas@xxxxxxxxxxx> > wrote:
> > > >
> > > >         Hi,
> > > >         Is it possible to run xen in a hvm guest which
> > > > itself is running on xen?
> > > >
> > > >
> > > >     Yes, you can.
> > > >
> > > >
> > > > Just to add only on hvm guests and not  on pv guests.
> > >
> > > And you can't run HVM guests inside the "xen on top of xen",
> > > as the HVM
> > > feature is not (in current CPU's) nestable.
> > >
> > > --
> > > Mats
> > >
> > > >     Thanks
> > > >     ~psr
> > > >
> > > >
> > > >
> > > >
> > > >         Thanks
> > > >
> > > >
> > > > ________________________________
> > > >
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> > > >
> > > >
> > > >
> > > >
> > > >
> > > >     --
> > > >     ---
> > > >     pradeep singh rautela
> > > >
> > > >     "Genius is 1% inspiration, and 99% perspiration" - not me :)
> > > >
> > > >
> > > >
> > > >
> > > > --
> > > > ---
> > > > pradeep singh rautela
> > > >
> > > > "Genius is 1% inspiration, and 99% perspiration" - not me :)
> > >
> > > ________________________________
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> >
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> 
> 
> 
> -- 
> 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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