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

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Subject: Re: [Xen-users] paravirt_ops DomU, does it mean performance?
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 11 Dec 2007 18:47:47 +0000
Cc: Emre Erenoglu <erenoglu@xxxxxxxxx>
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> I was wondering if using a paravirt_ops enabled kernel (say 2.6.23) as a
> Paravirtual DomU would mean higher performance out of the box, or if we
> still need to compile some xen specific device drivers for network and
> block devices.

The paravirt_ops kernel should run with similar performance to the XenLinux 
kernel from the main Xen tree.  It includes paravirtualisation for memory, 
SMP, etc as well as paravirtualised network and block devices.  It doesn't 
include support for suspend/resume, paravirt framebuffer, etc.

> Similarly, if HVM domains can run at the same performance as PV domains
> just by using PV drivers for network and block, why are we bothering
> ourselves with PV?  Why don't we -always- use HVM DomU's with PV drivers?

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.  PV can also run without hardware support, which is useful for some 

However, HVM solutions from Intel and AMD are going to keep getting more 
features and more optimisation.  Eventually they're going to do things so 
fast, and offer so many capabilities that we're probably going to want to use 
HVM everywhere - at least some parts of it.  There's no reason we can't 
combine HVM and PV techniques though...  in principle we could get a PV guest 
to run in an HVM so that we could use some combination of PV and HVM 
features.  Folks from Intel have been looking at something they call "hybrid 
virtualisation" which combines techniques from both PV and HVM.


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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