linux-kernel - Re: [PATCH] romsignature/checksum cleanup

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Date:	Sun, 07 Jan 2007 10:07:08 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rene Herman <rene.herman@...il.com>
CC:	Zachary Amsden <zach@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] romsignature/checksum cleanup

Rene Herman wrote:
> Doing the set_fs() and pagefault_{disable,enable} calls for every
> single byte during the checksum seems rather silly.

Why?  It's a bit of a performance hit, but that doesn't matter here. 
probe_kernel_address() is semantically the right thing to be using;
open-coding its contents to avoid a few fairly cheap operations is a
backwards step.

> I disagree I'm afraid. Given what __get_user compiles to (nothing more
> than a .fixup entry, basically) they're largely "free" and it makes
> the code completely obvious: "If you're touching this, do so via
> __get_user and not directly" and frees it from any assumptions,
> however reasonable or unreasonable.

My point is that "__get_user" doesn't make much semantic sense here:
we're not talking about usermode pages.  We used to use it quite often
for cases where an access may or may not fault, but now we spell that
"probe_kernel_address()".

> Would you _mind_ if I submit it? If not, if you could comment on
> whether or not these pagefault calls are still useful, that would be
> great.

I don't strongly object to using probe_kernel_address() for all ROM
memory accesses if it makes you feel happier, but I think putting an
open-coded implementation in here is definitely the wrong thing to do.

    J
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