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

[Sadique Puthen <sputhenp@xxxxxxxxxx>]

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?
--Sadique

>  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.
> Cheers,
> Mark
>
>   

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