linux-kernel - Re: Use of barriers in pvclock ABI

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Message-ID: <48A04968.7000805@qumranet.com>
Date:	Mon, 11 Aug 2008 17:15:04 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Gerd Hoffmann <kraxel@...hat.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Glauber de Oliveira Costa <glommer@...il.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm@...r.kernel.org>
Subject: Re: Use of barriers in pvclock ABI

Gerd Hoffmann wrote:
>   Hi,
>
>   
>> However, the pvclock_clocksource_read() implementation is
>> over-engineered, because it checks for an odd version and uses very
>> strong rmb() barriers (which generates either an "lfence" or "lock add
>> $0, (%esp)").
>>
>> If we're happy to guarantee as an ABI issue that the record will never
>> be updated cross-cpu, then we can make the barriers simply barrier() and
>> just check for (src->version != dst->version).
>>
>> Is that OK with you, or do you want to leave open the possibility of
>> doing cross-cpu time updates?
>>     
>
> Due to the TSC being involved here I don't expect cross-cpu time updates
> will ever happen.  IMHO it is fine to change that.
>
>   

I agree.  And if we ever feel the need, we can allocate a feature bit 
for it.

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

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