linux-kernel - Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

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Date:	Wed, 07 Mar 2007 01:40:07 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Dan Hecht <dhecht@...are.com>, Zachary Amsden <zach@...are.com>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	ak@...e.de,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>
Subject: Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

On Tue, 2007-03-06 at 16:24 -0800, Jeremy Fitzhardinge wrote:
> >> 3) clockevent set_next_event interface is suboptimal for paravirt (and 
> >> probably realtime-ish uses).  The problem is that the expiry is passed 
> >> as a relative time.  On paravirt, an arbitrary amount of (stolen) time 
> >> may have passed since the delta was computed and when the timer device 
> >> is programmed, causing that next interrupt to be too far out in the 
> >> future.  It seems a better interface for set_next_event would be to pass 
> >> the current time and the absolute expiry.  Actually, I sent email to 
> >> Thomas and Ingo about this (and some other clockevents/hrtimer feedback) 
> >> in July 2006, but never heard back.  Thoughts?
> >>     
> >
> > There is no problem for realtime uses, as the reprogramming path is
> > running with local interrupts disabled. I can see the point for paravirt
> > and I'm not opposed to change / expand the interface for that. It might
> > be done by an extra clockevents feature flag, which requests absolute
> > time instead of relative time.
> >   
> 
> I'm not sure how much different it makes overall.  It's true that
> absolute time would be a more useful interface, but because the guest
> vcpu can be preempted at any time, we could miss the timeout
> regardless.  In Xen if you set a timeout for the past you get an
> immediate interrupt; I presume the clockevent code can deal with that?

Yep. You also can return -ETIME so it just works w/o an interrupt.

	tglx




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