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[Xen-devel] Re: [PATCH 10/11] Support VIRQ_TIMER and pvclock on HVM

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 10/11] Support VIRQ_TIMER and pvclock on HVM
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Wed, 26 May 2010 14:08:23 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Don Dutile <ddutile@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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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.

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