Re: [Xen-devel] Xen inside Xen with VMX?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> thanks to everyone who replied! What I was thinking of was something along
> the lines of Daniel's suggestion below, i.e. if the VMX architecture makes
> it possible to setup enough proper traps etc. so that you can actually
> "cheaply" emulate all these VMX registers/features and thus have multiple,
> nested HVM's.

I wasn't able to find if it's possible to trap VMX instructions that are 
executed in noon-root mode but it's my assumption that Intel did the sensible 
thing and made that work :-)  They definitely cause a trap in AMD's SVM.

> I don't know if it's useful but it is at least interesting to 
> think about how it would work and how big/small a performance cost you
> could get away with if you had, say, 10 nested HVM's with Linux or
> something running in the innermost ;) I guess the emulation cost will be
> bigger for the "non-primary" HVM's (the primary being the one running
> closest to the hardware). But I guess that even stuff like VT-d directed
> I/O could be recursively emulated and yet still have "true" direct I/O for
> the innermost operating system.

Yeah, that sounds plausible.  Similarly, with nested pagetables / hardware 
assisted paging, you'd save overhead relative to nesting n shadow pagetables.

It'd be interesting whether by emulating these virtualisation-friendly 
features it might be possible to do nested full virtualisation without 
completely destroying performance...  I guess it'd entail a fair bit of scary 
coding to make it work but it would be really cool.

It could certainly be useful for testing stuff if nothing else.  I already do 
development and testing by running PV Linux on Xen, all wrapped up in a Xen 
HVM domain on my test box.  It works quite well and the performance is usable 
for testing / debugging.

I think Qemu can now emulate VMX / SVM instructions when it's run as an 
emulator (not as a virtualiser), so anyone who really needs virtual machines 
with HVM support right now can get it that way - if they don't mind taking a 
massive performance hit.

Cheers,
Mark

> MH > From: stodden@xxxxxxxxxx> To: bguthro@xxxxxxxxxxxxxxx> Date: Thu, 31
> Jan 2008 14:33:46 +0100> Subject: Re: [Xen-devel] Xen inside Xen with VMX?>
> CC: xen-devel@xxxxxxxxxxxxxxxxxxx; paradigm__82@xxxxxxxxxxx> > > On Thu,
> 2008-01-31 at 07:07 -0500, Ben Guthro wrote:> > Current VT implementations
> do not have this ability, AFAIK.> > Indeed. There's only one root mode, and
> the processor has no concept of> recursively stacking roots and
> accompanying protection levels on top of> each other.> > Nonetheless, root
> mode could be emulated, i.e. via shadow VMCBs,> optional shadow NPTs and
> emulation of the respective instruction subset.> > Would suffer from the
> same (most probably solvable) problems regarding> privilege compression and
> the like, but probably an interesting> excercise.> > Maybe one should put
> it on some (nonexisting) list for interesting> [academic] projects.> >
> regards,> daniel> > -- > Daniel Stodden> LRR - Lehrstuhl fÃr Rechnertechnik
> und Rechnerorganisation> Institut fÃr Informatik der TU MÃnchen D-85748
> Garching> http://www.lrr.in.tum.de/~stodden mailto:stodden@xxxxxxxxxx> PGP
> Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B> > > >
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