linux-kernel - Re: [rfc 08/45] cpu alloc: x86 support

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Date:	Tue, 20 Nov 2007 13:06:14 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Lameter <clameter@....com>
CC:	Andi Kleen <ak@...e.de>, akpm@...ux-foundation.org, travis@....com,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc 08/45] cpu alloc: x86 support

Christoph Lameter wrote:
> On Tue, 20 Nov 2007, Andi Kleen wrote:
> 
>>> This limitation shouldn't apply to the percpu area, since gs_base can be 
>>> pointed anywhere in the address space -- in effect we're always indirect.
>> The initial reference copy of the percpu area has to be addressed by
>> the linker.
> 
> Right that is important for the percpu references that can be folded by 
> the linker in order to avoid address calculations.
> 
>> Hmm, in theory since it is not actually used by itself I suppose you could 
>> move it into positive space.
> 
> But the positive space is reserved for a processes memory.
> 

But you wouldn't actually *use* this address space.  It's just for the 
linker to know what address to tag the references with; it gets 
relocated by gs_base down into proper kernel space.  The linker can 
stash the initialized reference copy at any address (LMA) which can be 
different from what it will be used at (VMA); that is not an issue.

To use %rip references, though, which are more efficient, you probably 
want to use offsets that are just below .text (at -2 GB); presumably
-2 GB-[max size of percpu section].  Again, however, no CPU actually 
needs to have its data stashed in that particular location; it's just an 
offset.

	-hpa
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