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Re: [Xen-users] paravirt_ops DomU, does it mean performance?

To: Sadique Puthen <sputhenp@xxxxxxxxxx>
Subject: Re: [Xen-users] paravirt_ops DomU, does it mean performance?
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 11 Dec 2007 19:57:40 +0000
Cc: xen-users@xxxxxxxxxxxxxxxxxxx, Emre Erenoglu <erenoglu@xxxxxxxxx>
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> > I suspect the performance of PV domains ought still to be a bit better
> > than for HVM - at least for some workloads.  PV eliminates the need for
> > shadow pagetables and reduces the number of things that need to be
> > trapped and emulated.
> Is it still true to say that HVM uses shadow pagetables in the light of
> AMD nested pagetables?  Or is my understanding wrong?

It's true that HVM doesn't always have to be shadow-pagetable based.  I'm not 
clear whether AMD NPT-capable hardware is on sale yet (it was in the original 
SVM spec but not shipped in the first SVM processors).  Intel have brought 
out hardware assisted paging specs also, so we should soon not require shadow 
PTs on Intel hardware either.  Most HVM hardware currently out there has to 
use shadow PTs, though.

This is an example of something we might want to use for PV guests in Xen, if 
it gets fast enough, in order to avoid trapping into Xen for pagetable 
updates.  This should be doable, even for older guests, since as long as they 
support the "hypercall transfer page" then they can potentially have their 
hypercall implementations redefined when they are loaded.


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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