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

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Date:	Wed, 07 Mar 2007 01:49:04 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dan Hecht <dhecht@...are.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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 16:35 -0800, Dan Hecht wrote:
> >> 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?
> > 
> 
> That's the problem though, you won't know to set it for the past since 
> the expiry is relative.  When the vcpu starts running again, it will set 
> the timer to expire X ns from now, not Xns from when the timer was 
> requested.

Ooops. I completely forgot, that you get the absolute expiry time
already in ktime_t format (nanoseconds) when dev->set_next_event() is
called.

	dev->next_event = expires;

is done right before the call. 

So it's already there for free.

	tglx


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