linux-kernel - Re: kvm & dyntick

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Date:	Sun, 14 Jan 2007 09:39:54 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...ranet.com>
Cc:	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: kvm & dyntick


* Avi Kivity <avi@...ranet.com> wrote:

> >( for this to work on my system i have added a 'hyper' clocksource
> >  hypercall API for KVM guests to use - this is needed instead of the 
> >  running-to-slowly TSC. )
> >  
> 
> What's the problem with the TSC?  The only issue I'm aware of is that 
> the tsc might go backwards if the vcpu is migrated to another host cpu 
> (easily fixed).

this is not a problem of KVM - this is a problem of this laptop: its TSC 
stops when going idle. So the TSC is fundamentally unusable for reliable 
timekeeping - and qemu doesnt offer pmtimer emulation to fall back to, 
so the dyntick kernel refused to go high-res under KVM. Once i added the 
hyper clocksource to fall back to, it could utilize the PIT and the 
lapic for clock-events.

	Ingo
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