linux-kernel - Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel

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Message-ID: <45176865.7020300@goop.org>
Date:	Sun, 24 Sep 2006 22:25:57 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Andi Kleen <ak@....de>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.osdl.org>
Subject: Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel

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