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Date: Wed, 26 May 2010 14:08:23 +0100
From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
       Don Dutile <ddutile@redhat.com>
Subject: Re: [PATCH 10/11] Support VIRQ_TIMER and pvclock on HVM
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On Tue, 25 May 2010, Jeremy Fitzhardinge wrote:
> On 05/24/2010 11:27 AM, Stefano Stabellini wrote:
> > Use the paravirtualized timer (VIRQ_TIMER) as timer and the
> > paravirtualized clock (pvclock) as wallclock on HVM domains too.
> >   
> 
> That's not quite correct.  There's the Xen clockevent driver, the Xen
> clocksource, and a distinct-but-related Xen notion of wallclock time. 
> In general, clocksources and wallclock don't have much direct
> relationship to each other.

Let me rephrase it:

use xen_vcpuop_clockevent instead of hpet and APIC timers as main
clockevent device on all vcpus, use the xen wallclock time as wallclock
instead of rtc and use xen_clocksource as clocksource.
The pv clock algorithm needs to work correctly for the xen_clocksource
and xen wallclock to be usable, only modern Xen versions offer a
reliable pv clock in HVM guests (XENFEAT_hvm_safe_pvclock).

> 
> > Any interaction with emulated hardware is not very fast, so using the
> > paravirtualized timer instead of the emulated hpet is defenitely a
> >   
> "definitely"
> 
> > performance improvement.
> >   
> 
> Are there accuracy/stability/precision improvements too?
> 

Using the hpet as clocksource means a VMEXIT every time we read/write to
the hpet mmio addresses, pvclock give us a better rating without
VMEXITs. Same goes for the xen wallclock and xen_vcpuop_clockevent.