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Re: [Xen-users] Re: Why pv-on-hvm drivers?

On Mon, Sep 06, 2010 at 10:59:26AM +0200, Markus Schuster wrote:
> Pasi Kärkkäinen wrote:
> > On Sun, Sep 05, 2010 at 01:29:19AM +0200, Markus Schuster wrote:
> >> Hi list,
> >> 
> >> I've read about recent efforts to push pv-on-hvm drivers to Linux
> >> mainline and I'm curious to know the cause for this. What's the advantage
> >> over using pv_ops directly and booting the kernel paravirtualized?
> > 
> > The other point is performance: 32bit PV (paravirtualized) guests
> > perform OK, but 64bit PV guests have a performance hit if your
> > workload creates a lot of new processes in the guest.
> > 
> > HVM helps there; 64bit Linux guests might be faster as HVM,
> > depending on the workload.
> Hi Parsi, thanks for your (as usual :)) good answer. 
> That's the first time I read about a PV performance hit compared to HVM - 
> maybe you (or someone else) can write a few words about what's causing that? 
> Could be interesting for other people, maybe?

I think there are some XenSummit presentations about it on xen.org website.
It has to do with 32bit vs 64bit architecture differences related to memory 

Every time a new process is created by the 64bit PV kernel
the guest process pagetables need to be verified/checked by the hypervisor,
and this causes a performance hit if you need to create a lof 
of new processes in the guest.

It doesn't affect 'long running' processes in a 64bit PV guest,
ie. the performance hit happens only when new processes 
are created often (kernel compilation, unixbench).

For an HVM guest that stuff is handled by the CPU/hardware,
so there's no performance hit related to it. 
HVM guests have some other performance hits though..

That's my understanding of it :)

-- Pasi

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