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

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Date:	Tue, 06 Mar 2007 16:35:52 -0800
From:	Dan Hecht <dhecht@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	tglx@...utronix.de, 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 03/06/2007 04:24 PM, Jeremy Fitzhardinge wrote:
> Thomas Gleixner 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?
> 

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.
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