From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755277Ab1LMOvh (ORCPT ); Tue, 13 Dec 2011 09:51:37 -0500 Received: from www.linutronix.de ([62.245.132.108]:57638 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754869Ab1LMOvc (ORCPT ); Tue, 13 Dec 2011 09:51:32 -0500 Date: Tue, 13 Dec 2011 15:51:26 +0100 (CET) From: Thomas Gleixner To: Avi Kivity cc: Ingo Molnar , Pekka Enberg , LKML , Sasha Levin Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul In-Reply-To: <4EE75FF8.5060208@redhat.com> Message-ID: References: <20111210132220.083204833@linutronix.de> <20111211103047.GA19299@elte.hu> <20111211155347.GA1297@elte.hu> <20111212172059.GC23870@elte.hu> <20111212192135.GA18876@elte.hu> <4EE72FCB.3070109@redhat.com> <4EE75FF8.5060208@redhat.com> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 13 Dec 2011, Avi Kivity wrote: > On 12/13/2011 03:52 PM, Thomas Gleixner wrote: > > On Tue, 13 Dec 2011, Avi Kivity wrote: > > > Run your guest with x2apic enabled, the timing will be very different. > > > > And what magic do I have to use to make that happen other than having > > x2apic support enabled in the kernel? Or do I need a certain kernel > > version for host and guest to make that work? > > With qemu, -cpu host or -cpu blah,+x2apic. kvm-tool does the equivalent Well, that was a trace from qemu-kvm (your latest tree). Now with -cpu host it works and I get the same results as with kvm tool. > of -cpu host, so I'm surprised it doesn't show up. x2apic has been > recognized by the guest for a long time (ce69a784; 2.6.32, I'll be > surprised if you have anything older than that on your machine). > > Does x2apic show up in your guest's /proc/cpuinfo? Now it does :) > > The whole APIC timer calibration and the back and forth > > conversion is definitely nothing which falls into the category of > > smart. > > APIC timer calibration is silly but it hasn't proven to be a real-world > problem. Well, that depends on the world. The issue with that calibration is that it can be off and for a certain category of application it matters. Definitely not for the usual virt workload stuff. But saving cycles for highres/nohz enabled guests would be nice. Maybe I have a go when I find a some time. Thanks, tglx