Xen devel - Apr 2006 - Different instruction stream for PV OS on VT/Pacifica?

I was asked a question and I think I know the answer for
Xen/ia64 but am not sure I know the answer for Xen/x86 --
or if there is a different answer for VT vs Pacifica.

In the three scenarios:
A. Paravirtualized (PV) OS running on a pre-HVM processor
   using a virtualization-capable Xen
B. PV OS running on a VT (or Pacifica) processor using a
   virtualization-capable Xen but NOT running any fully
   virtualized (FV) OS
C. PV OS running on a VT (or Pacifica) processor using a
   virtualization-capable Xen which is sharing the processor
   with a FV OS

is the sequence of instructions executed by the processor
on behalf of the PV OS -- including instructions in firmware
and in Xen -- identical or different/faster/slower?  (Ignoring
irrelevant factors such as domain switch, non-virtualization-
related processor differences, etc.)

On Xen/ia64, a PV OS should always execute the same
sequence of instructions, whether or not it is on
a VT-capable processor.

Can experts for the others (VT-x and Pacifica) comment?

Thanks,
Dan


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

> -----Original Message-----
> From: xen-devel-bounces@lists.xensource.com 
> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of 
> Magenheimer, Dan (HP Labs Fort Collins)
> Sent: 28 April 2006 17:27
> To: xen-devel
> Subject: [Xen-devel] Different instruction stream for PV OS 
> on VT/Pacifica?
> 
> I was asked a question and I think I know the answer for
> Xen/ia64 but am not sure I know the answer for Xen/x86 -- or 
> if there is a different answer for VT vs Pacifica.
> 
> In the three scenarios:
> A. Paravirtualized (PV) OS running on a pre-HVM processor
>    using a virtualization-capable Xen
> B. PV OS running on a VT (or Pacifica) processor using a
>    virtualization-capable Xen but NOT running any fully
>    virtualized (FV) OS
> C. PV OS running on a VT (or Pacifica) processor using a
>    virtualization-capable Xen which is sharing the processor
>    with a FV OS
> 
> is the sequence of instructions executed by the processor on 
> behalf of the PV OS -- including instructions in firmware and 
> in Xen -- identical or different/faster/slower?  (Ignoring 
> irrelevant factors such as domain switch, non-virtualization- 
> related processor differences, etc.)
> 
> On Xen/ia64, a PV OS should always execute the same sequence 
> of instructions, whether or not it is on a VT-capable processor.
> 
> Can experts for the others (VT-x and Pacifica) comment?
I''d say that this answer applies to AMD processors (and as far as I
know
it applies to Intel too - but I''m not really a spokesperson for AMD,
and
even less so for Intel.)

There''s been some thinking that we could use HVM (VT/SVM (pacifica)) to
enhance PV guest too, but so far nothing has been implemented [and I
guess once it does get implemented, IA64 will follow shortly afterwards,
assuming it''s generally useful and not specific to x86]. 

--
Mats
> 
> Thanks,
> Dan
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
> 
> 

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel