linux-kernel - Re: [PATCH -mm 0/4] x86_64 boot: Add linked listof struct setup

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Date:	Thu, 27 Mar 2008 10:46:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Jackson <pj@....com>
Cc:	ying.huang@...el.com, hpa@...or.com, andi@...stfloor.org,
	mingo@...hat.com, tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 0/4] x86_64 boot: Add linked listof struct
	setup_data to boot protocol

* Paul Jackson <pj@....com> wrote:

> Ingo wrote:
> > stupidly shortsighted limit ...
> 
> Eh ... I don't think all that shortsighted ... that 4096 limit has 
> probably been around longer than Linux.  Linus or hpa or others would 
> know the history better than I do.

the limit for boot parameters is 2048 bytes - the BUILD_WARN_ON() checks 
for the sum of the two boot parameter areas. We used empty_zero_page for 
boot parameter passing for a near eternity - 2K for the command line 
(which used to be clipped - recently upped to true 2048), and 2K for the 
boot parameters.

As a historic/nostalgic sidenote: the 2048 limit is not as old as Linux, 
version 0.11 of Linux had 512 bytes of special area for boot parameters, 
at 0x90000-0x901FF.

having fixed-length 2K buffering ABI between two historically rather 
inflexible pieces of software (the bootloader and the kernel), at a very 
fragile and under-capable junction of our bootstrap (very early during 
bootup) _is_ shortsighted. Granted, it's a difficult area to change but 
still it's a nasty limit - as you have found it out first-hand ;-)

	Ingo
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