Message-ID: <4981FA0B.5010101@zytor.com>
Date: Thu, 29 Jan 2009 10:48:43 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Christoph Lameter <cl@linux-foundation.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Rusty Russell <rusty@rustcorp.com.au>, Tejun Heo <tj@kernel.org>,
       Ingo Molnar <mingo@elte.hu>, Herbert Xu <herbert@gondor.apana.org.au>,
       akpm@linux-foundation.org, brgerst@gmail.com, ebiederm@xmission.com,
       travis@sgi.com, linux-kernel@vger.kernel.org, steiner@sgi.com,
       hugh@veritas.com, "David S. Miller" <davem@davemloft.net>,
       netdev@vger.kernel.org
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
References: <20090115183942.GA6325@elte.hu> <200901271213.18605.rusty@rustcorp.com.au> <497E705B.5000302@kernel.org> <200901282108.51864.rusty@rustcorp.com.au> <alpine.DEB.1.10.0901281148540.13825@qirst.com> <20090128180757.GA9908@Krystal> <alpine.DEB.1.10.0901291315320.9622@qirst.com>
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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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