linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Date:	Wed, 09 Jul 2008 20:19:52 -0400
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
> 
>>> Which means that my idea of using the technique we use on x86_32 will not
>> work.
>>
>> No, the compiler memory model we use guarantees that everything will be within
>> 2G of each other.  The linker will spew loudly if that's not the case.
> 
> The per cpu area is at least theoretically dynamically allocated.  And we
> really want to put it in cpu local memory.    Which means on any reasonable
> NUMA machine the per cpu areas should be all over the box.
> 
> So there is no guarantee that with an arbitrary 64bit address in %gs of anything.
> 

That doesn't matter in the slightest.

> Grr.  Except you are correct.  We have to guarantee that the offsets we have
> chosen at compile time still work.  And we know all of the compile time offsets
> will be in the -2G range.  So they are all 32bit numbers.  Negative 32bit
> numbers to be sure.  That trivially leaves us with everything working except
> the nasty hard coded decimal 40.

The *offsets* have to be in the proper range, but the %gs_base is an 
arbitrary 64-bit number.

	-hpa

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