linux-kernel - Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

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Date:	Wed, 30 Aug 2006 10:13:35 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <ak@...e.de>
CC:	Chuck Ebbert <76306.1226@...puserve.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH RFC 0/6] Implement per-processor data areas for  i386.

Andi Kleen wrote:
> I tried that once on x86-64, but it wasn't possible because the linker
> is missing the right relocations. It has something on the first look similar 
> for __thread data in user space, but it wasn't usable for the kernel.
>   
The other difficulty is that you can't take the addresses of things in 
the pda and pass them around, which happens a lot.  It would be another 
matter if you could add an attribute to a pointer which told gcc "always 
use a %gs override for indirecting through this pointer", though that 
would still require some type qualifier on any pointer which holds the 
value.

(Hm, it would be interesting to see if we could possibly use the code 
generated by gcc for TLS variables...)

> Even with a single indirection it is still far more efficient than a 
> array lookup.

Yes.  Even in the Xen case it will be useful to have a pointer to the 
vcpu structure, at least until Xen can be convinced to put the vcpu 
structure in the PDA itself.

    J
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