linux-kernel - Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

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Message-ID: <4AE986FE.3040104@redhat.com>
Date:	Thu, 29 Oct 2009 14:13:50 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Glauber Costa <glommer@...hat.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	kurt.hackel@...cle.com, the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir.fraser@...citrix.com>, zach.brown@...cle.com,
	chris.mason@...cle.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall	implementation

On 10/28/2009 07:47 PM, Jeremy Fitzhardinge wrote:
>> Much better to have an API for this.  Life is hacky enough already.
>>      
> My point is that if an app cares about property X then it should just
> measure property X.  The fact that gettimeofday is a vsyscall is just an
> implementation detail that apps don't really care about.  What they care
> about is whether gettimeofday is fast or not.
>    

But we can not make a reliable measurement.

> If the environment has such unstable timing that the effect can't be
> measured, then it is moot whether its a vsyscall or not (but in that
> case its almost certainly better to use the standard API rather than
> trying to roll your own timesource with rdtsc).
>    

If you're interested in gettimeofday() for a global monotonic counter 
you can fall back to atomic_fetch_and_add() which will be faster than a 
syscall even on large systems.  Maybe we should provide a vsyscall for 
global monotonic counters and implement it using a atomics or tsc 
instead of these hacks (I'm assuming here that the gettimeofday() calls 
are used to implement an atomic counter - are they?)

-- 
error compiling committee.c: too many arguments to function

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