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

To: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Subject: Re: [Xen-users] paravirt_ops DomU, does it mean performance?
From: Sadique Puthen <sputhenp@xxxxxxxxxx>
Date: Wed, 12 Dec 2007 00:26:44 +0530
Cc: xen-users@xxxxxxxxxxxxxxxxxxx, Emre Erenoglu <erenoglu@xxxxxxxxx>
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Mark Williamson wrote:
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.

Is it still true to say that HVM uses shadow pagetables in the light of AMD nested pagetables? Or is my understanding wrong?


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

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.


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