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Re: [Xen-devel] pv_ops dom0 clock only runs under load

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] pv_ops dom0 clock only runs under load
From: Milan Holzäpfel <listen@xxxxxxxx>
Date: Fri, 6 Mar 2009 23:49:12 +0100
Cc: Milan Holzäpfel <mail@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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On Fri, 06 Mar 2009 13:56:42 -0800
Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

> > Booting with idle=poll appended to the kernel command line makes the
> > TSC work again. 
> > [...]
> No, its not a great solution.  But a tsc which only runs while the cpu 
> is non-idle isn't a huge problem; it just needs Xen to update the timing 
> parameters each time it idles the CPU.  But its a bit of a mystery that 
> works OK...

Probably some changes in the rest of the kernel?  The "TSC halts in
idle" message didn't appear with 2.6.18.  The behaviour of the TSC is
the same with 2.6.18/2.6.29 on bare metal (works/doesn't work).
Maybe only some idle states (which weren't used in 2.6.18) halt the
TSC, but that's only guesswork on my side. 

Anyway, I will be glad to test a fix as soon as there is one.  

> > Another question: The xm dmesg I posted reports "(XEN) Platform timer
> > is 14.318MHz HPET".  Why is the TSC used for time keeping then? The
> > HPET works perfectly on every kernel not running under Xen. 
> >   
> Xen uses the hpet to maintain a notion of system-wide time.  However its 
> fairly expensive to read the hpet, so each CPU uses its own local time 
> which is tracked using the tsc and synchronized with the hpet.

Ah, that's interesting. 


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