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Date: Sun, 14 Jan 2007 09:39:54 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Avi Kivity <avi@qumranet.com>
Cc: kvm-devel <kvm-devel@lists.sourceforge.net>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>
Subject: Re: kvm & dyntick
Message-ID: <20070114083954.GB2913@elte.hu>
References: <45A66106.5030608@qumranet.com> <20070112062006.GA32714@elte.hu> <20070112101931.GA11635@elte.hu> <45A7BF9F.5090508@qumranet.com>
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* Avi Kivity <avi@qumranet.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