From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel
Date: Sun, 24 Sep 2006 18:03:04 -0700
Message-ID: <45172AC8.2070701@goop.org>
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To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andi Kleen <ak@muc.de>, lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>, virtualization <virtualization@lists.osdl.org>
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Rusty Russell wrote:
>> So are symbols referencing the .data.percpu section 0-based?  Wouldn't =

>> you need to subtract __per_cpu_start from the symbols to get a 0-based =

>> segment offset?
>>     =

>
> I don't think I understand the question.
>
> The .data.percpu section is the "template" per-cpu section, freed along
> with other initdata: after setup_percpu_areas() is called, it is not
> supposed to be used.  Around that time, the gs segment is set up based
> at __per_cpu_offset[cpu], so "%gs:<varname>" accesses the local version.
>   =


If you do

    DEFINE_PER_CPU(int, foo);

then this ends up defining per_cpu__foo in .data.percpu.  Since =

.data.percpu is part of the init data section, it starts at some address =

X (not 0), so the real offset into the actual per-cpu memory is actually =

(per_cpu__foo - __per_cpu_start).  setup_per_cpu_areas() builds this =

delta into the __per_cpu_offset[], and so it means that the base of your =

%gs segment is at -__per_cpu_start from the actual start of the CPU's =

per-cpu memory, and the limit has to be correspondingly larger.  Which =

is a bit ugly.  Especially since "__per_cpu_start" is actually very =

large, and so this scheme pretty much relies on being able to wrap =

around the segment limit, and will be very bad for Xen.

An alternative is to put the "-__per_cpu_start" into the addressing mode =

when constructing the address of the per-cpu variable.

    J