linux-kernel - Re: [rfc 37/45] x86_64: Support for fast per cpu operations

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Message-ID: <4742435F.6010803@zytor.com>
Date:	Mon, 19 Nov 2007 18:15:59 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Lameter <clameter@....com>
CC:	ak@...e.de, akpm@...ux-foundation.org, travis@....com,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc 37/45] x86_64: Support for fast per cpu operations

Christoph Lameter wrote:
> On Mon, 19 Nov 2007, H. Peter Anvin wrote:
> 
>> There was, at some point, discussion about using the gcc TLS mechanism, which
>> should permit even better code to be generated.  Unfortunately, it would
> 
> How would that be possible? Oh. You mean the discussion where I mentioned 
> using the thread attribute?
> 
>> require gcc to be able to reference %gs instead of %fs (and vice versa for
>> i386), which I don't think is available in anything except maybe the most
>> cutting-edge version of gcc.
> 
> Right. That is why we do it in ASM here.
> 
>> However, if we're doing a masssive revampt it would be good to get an idea of
>> how to migrate to that model eventually, or why it doesn't make sense at all.
> 
> If you can tell me what the difference would be then we can discuss it. 
> AFAICT there is no difference. Both use a segment register.
> 

As far as I can tell from a *very* brief look at your code (which means 
I might have misread it), these are the differences:

- gcc uses %fs:0 to contain a pointer to itself.
- gcc uses absolute offsets from the thread pointer, rather than adding
   %rip.  The %rip-based form is actually more efficient, but it does
   affect the usable range off the base pointer.

	-hpa
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