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Date: Thu, 01 Oct 2009 09:06:33 +1000
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From: Peter Chubb <peterc@gelato.unsw.edu.au>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: Christoph Lameter <cl@linux-foundation.org>, Tejun Heo <teheo@suse.de>,
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       Rusty Russell <rusty@rustcorp.com.au>,
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Subject: Re: [PATCHSET percpu#for-next] percpu: convert ia64 to dynamic percpu and drop the old one, take#2
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>>>>> "Tony" == Tony Luck <Luck> writes:

>> Tony: Could we use a global register for the per cpu address? That
>> would make IA64 work similar to sparc.

Tony> Would that be a useful trade of resources for convenience?
Tony> We've already hard-wired r13 for "current". Grabbing another one
Tony> would require fixing (since user code will have clobbered it).
Tony> Possibly re-working any existing code that already uses whatever
Tony> register you choose.

r3, r4 and r5 are currently unused by the kernel, and unused
by GCC and ICC.   Only hand-written assembler and weird compilers use
those registers(and my virtual-machine monitor :-().  If you wanted to
experiment, that'd be a starting place.  

I'm not sure of the advantage though -- TLB mapping is relatively
cheap, and we're no longer hard-wiring the translation register.

You';d have to do somne careful benchmarking on a wide variety of
workloads and machines to get a definitive answer.


---
Dr Peter Chubb        www.nicta.com.au      peter DOT chubb AT nicta.com.au
http://www.ertos.nicta.com.au           ERTOS within National ICT Australia
>From Imagination to Impact                       Imagining the (ICT) Future
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