linux-kernel - Re: [patch 0/3] kvm tool: Serial emulation overhaul

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Date:	Tue, 13 Dec 2011 15:51:26 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Avi Kivity <avi@...hat.com>
cc:	Ingo Molnar <mingo@...e.hu>, Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Sasha Levin <levinsasha928@...il.com>
Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul

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
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