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

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Date:	Tue, 20 Nov 2007 12:43:10 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <ak@...e.de>
CC:	Christoph Lameter <clameter@....com>, 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

Andi Kleen wrote:
> On Tuesday 20 November 2007 04:50, Christoph Lameter wrote:
>> On Tue, 20 Nov 2007, Andi Kleen wrote:
>>> I might be pointing out the obvious, but on x86-64 there is definitely
>>> not 256TB of VM available for this.
>> Well maybe in the future.
> 
> That would either require more than 4 levels or larger pages
> in page tables.
> 
>> One of the issues that I ran into is that I had to place the cpu area
>> in between to make the offsets link right.
> 
> Above -2GB, otherwise you cannot address them
> 

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.

Obviously the offsets *within* the percpu area has to be in range (±2 GB 
per cpu for absolute offsets, slightly smaller for %rip-based addressing 
-- obviously judicious use of an offset for gs_base is essential in the 
latter case).

Thus you want the percpu areas below -2 GB where they don't interfere 
with modules or any other precious address space.

	-hpa
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