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From: Andi Kleen <ak@suse.de>
To: Christoph Lameter <clameter@sgi.com>
Subject: Re: broken local_t on i386
Date: Mon, 12 Jun 2006 19:35:02 +0200
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Cc: Ingo Molnar <mingo@elte.hu>,
       Michal Piotrowski <michal.k.k.piotrowski@gmail.com>,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
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On Monday 12 June 2006 20:27, Christoph Lameter wrote:
> On Mon, 12 Jun 2006, Andi Kleen wrote:
> > It does, but the per cpu data that everybody uses doesn't reside in the
> > PDA because it wasn't possible to make this work with binutils
> >
> > It would require a relocation relative to another symbol which isn't
> > really supported.
>
> I dont think you need a relocation relative to another symbol. Map the
> pda to a virtual adress range. That is then translated with a
> processor specific page table to various physical addresses.

Per processor page tables are not really practical on x86-64. They would
get really messy because you would need to duplicate the top level
of the page tables per CPU.

>
> > At some point I considered using runtime patching to work around
> > this limitation, but it would be some work and relatively complex.
>
> Well this would drastically decreased the overhead for PDA access and fix
> local.t

Saving two instructions? And PDA is usually in L1. I doubt you could benchmark
a difference.

-Andi