linux-kernel - Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel

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Message-ID: <20060922123215.GA98728@muc.de>
Date:	Fri, 22 Sep 2006 14:32:15 +0200
From:	Andi Kleen <ak@....de>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.osdl.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel

BTW I changed my copy sorry. I redid the early PDA support
to not be in assembler.

On Fri, Sep 22, 2006 at 09:59:45PM +1000, Rusty Russell wrote:
> This patch actually uses the gs register to implement the per-cpu
> sections.  It's fairly straightforward: the gs segment starts at the
> per-cpu offset for the particular cpu (or 0, in very early boot).  
> 
> We also implement x86_64-inspired (via Jeremy Fitzhardinge) per-cpu
> accesses where a general lvalue isn't needed.  These
> single-instruction accesses are slightly more efficient, plus (being a
> single insn) are atomic wrt. preemption so we can use them to
> implement cpu_local_inc etc.

The problem is nobody uses cpu_local_inc() etc :/ And it is difficult
to use in generic code because of the usual preemption issues 
(and being slower on other archs in many cases compared to preempt disabling
around larger block of code) 

Without that it is the same code as Jeremy's variant
%gs memory reference + another reference with offset as far as I can see.

So while it looks nice I don't think it will have advantages. Or did
i miss something?

-Andi
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