linux-ext4 - Re: [PATCH 1/1] x86: fix text

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Message-ID: <y0my771fjou.fsf@ton.toronto.redhat.com>
Date:	Fri, 25 Apr 2008 22:12:33 -0400
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>, Jiri Slaby <jirislaby@...il.com>,
	David Miller <davem@...emloft.net>, zdenek.kabelac@...il.com,
	rjw@...k.pl, paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-ext4@...r.kernel.org, herbert@...dor.apana.org.au,
	penberg@...helsinki.fi, clameter@....com,
	linux-kernel@...r.kernel.org, pageexec@...email.hu,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 1/1] x86: fix text_poke

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> [...]
>> The point is to provide a way to dynamically enable code at runtime
>> without noticeable performance impact on the system.
>
> Quite frankly, maybe I'm a bit dense, but why don't you just recompile the 
> whole original function (at run-time), load that new version of a function 
> as a mini-module, and then insert a marker at the top of the old function 
> that just does a "jmp replacementfunction". [...]

You mentioned possible solutions to some of the problems this
ambitious an approach would cause.  Here are a few more complications:

- instrumenting inlined functions

- proper sharing of static function data amongst multiple live
  copies of same function

- unknown implications of violating long-standing assumptions about
  functions not changing addresses

- interaction with other code modification machinery (kprobes, ...)

- necessity to carry kernel sources & compilers on machines; slow
  marker activation


- FChE
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