linux-kernel - Re: [PATCH 0/2] x86-64: allow using RIP-relative addressing for per-CPU data

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Date:	Wed, 05 Nov 2014 17:09:36 +0000
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<luto@...capital.net>, <mingo@...e.hu>, <tglx@...utronix.de>,
	<hpa@...or.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] x86-64: allow using RIP-relative addressing
 for per-CPU data

>>> Andy Lutomirski <luto@...capital.net> 11/04/14 8:33 PM >>>
>On 11/04/2014 12:49 AM, Jan Beulich wrote:
>> Observing that per-CPU data (in the SMP case) is reachable by
>> exploiting 64-bit address wraparound, these two patches
>> arrange for using the one byte shorter RIP-relative addressing
>> forms for the majority of per-CPU accesses.
>> 
>> 1: handle PC-relative relocations on per-CPU data
>> 2: use RIP-relative addressing for most per-CPU accesses
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>> 
>
>I'm lost here.  Can you give an example of a physical and virtual
>address of an instruction, the address within the gs segment, and why
>the relocations are backwards?

When an instruction using RIP relative addressing gets moved up in
address space, the distance to the target address decreases. I.e. it's the
opposite of a normal, non-PC-relative base relocation (where the target
address increases together with the instruction getting moved up).

Jan

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