linux-kernel - Re: [PATCH 0/5] RFC: x86: Early exception table support

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Date:	Thu, 19 Apr 2012 11:22:55 +0200
From:	Borislav Petkov <bp@...64.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...64.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>
Subject: Re: [PATCH 0/5] RFC: x86: Early exception table support

Haha,

I was looking into that too and you beat me to it. Now isn't that great!
:-)

On Wed, Apr 18, 2012 at 05:16:45PM -0700, H. Peter Anvin wrote:
> If we get an exception during early boot, walk the exception table to
> see if we should intercept it.  The main use case for this is to allow
> rdmsr_safe()/wrmsr_safe() during CPU initialization.
> 
> Since the exception table is currently sorted at runtime, and fairly
> late in startup, this code walks the exception table linearly.  We
> obviously don't need to worry about modules, however: none have been
> loaded at this point.
> 
> In the future it would be better to have the table sorted at compile
> time, or even better, turned into a perfect hash.  At that point this
> code should be changed out from doing a linear search.

FWIW, I was thinking that maybe we could sort the main extable
(i.e., no modules) before we assign the early idt handlers in
x86_64_start_kernel() and this way use the binary search through it in
search_extable().

I'm sending the patches I had as a reply to this mail so that you can
get a better idea of what I mean. They're rough and the asm is f*cked
up so don't look at that - I was still figuring out the exception frame
layout to do it properly.

> This patchset also makes the early exception handling a little more
> similar between x86-64 and i386, but a lot of unification could (and
> should) still be done: in particular, setup done in C as in x86-64,
> and verbose error dump as in i386.

Yeah, I'll look through those and give them a test drive on my boxes
here.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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