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