linux-kernel - Re: [PATCH] percpu: add optimized generic percpu accessors

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Message-ID: <4981FA0B.5010101@zytor.com>
Date:	Thu, 29 Jan 2009 10:48:43 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org, brgerst@...il.com,
	ebiederm@...ssion.com, travis@....com,
	linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors

Christoph Lameter wrote:
> 
> gcc/glibc support a __thread attribute to variables. As far as I can tell
> this automatically makes gcc perform the relocation to the current
> context using a segment register.
> 
> But its a weird ABI http://people.redhat.com/drepper/tls.pdf. After
> reading that I agree that we should stay with the cpu ops and forget about
> the local and thread stuff in gcc/glibc.
> 

We have discussed this a number of times.  There are several issues with
it; one being that per-cpu != per-thread (we can switch CPU whereever
we're preempted), and another that many versions of gcc uses hardcoded
registers, in particular %fs on 64 bits, but the kernel *has* to use %gs
since there is no swapfs instruction.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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