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 22:25:57 -0700
Message-ID: <45176865.7020300@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:
> That can't happen, since 0xc100000 is not in the kernel address space.
> 0xc1000000 is though, perhaps that's what you meant?
>   =


Yes, it is.  Though it doesn't actually make any material difference to =

my argument.

>> So, in this case the %gs base will be loaded with 0xc100000-0xc0431100 =
=3D =

>> 0x4bccef00
>>     =

>
>   =

> A negative offset, exactly, which can't happen, as I said.
0x4bccef00 is positive. The correct number is 0xc1000000-0xc0431100 =3D =

0xbcef00

The %gs:per_cpu__foo addressing mode still calculates =

0xbcef00+0xc0433800, which is still a subtraction.  My essential point =

is that *all* kernel addresses (=3Dkernel symbols) are negative, so using =

them as an offset from a segment base (any segment base) is a =

subtraction, which requires a 4G limit.

    J