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Date: Thu, 25 Oct 2007 01:47:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Joseph Parmelee <jparmele@wildbear.com>
Cc: linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: Old version of lilo fails to boot 2.6.23
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On Tue, 16 Oct 2007 01:07:31 -0600 (CST) Joseph Parmelee <jparmele@wildbear.com> wrote:

> 
> Greetings:
> 
> I upgraded to version 2.6.23 and had a fun time figuring out the source of
> this boot failure message on my x86 system:
> 
>    This kernel requires an i<random integer>86 CPU, but only detected an
>    i<smaller random integer>86 CPU.
> 
> It turns out that my version of lilo (lilo -V gives version 21) doesn't set
> up the stack and data segment registers in a compatible manner before
> entering the new 16-bit real mode kernel loader code.  This problem is new
> to the 2.6.23 series.

hm, one of my test boxes runs

vmm:/home/akpm> lilo -V       
LILO version 21.4-4

and I haven't had any such problems.

> Parts of the 16-bit real mode loader code are now being compiled as C code
> with gcc in 32 bit mode passing the .code16gcc directive to the assembler to
> correct the stack frames to 16 bit.  This kludge won't work unless all the
> 16-bit segment registers are set to the same value.  Gcc only manipulates
> the offset of the address and doesn't know anything about segment registers
> or segment override prefixes.  My lilo was setting SS=0x8000, DS=0x9000, and
> SP=0xB000 before entering the kernel loader.  This makes stack automatics
> unreachable from the data segment without segment override prefixes.
> 
> I was tempted to patch the kernel code, but instead decided to try
> "upgrading" lilo to grub-0.97 and found that grub works just fine.  This
> also has the significant advantage that we won't need those nasty as86 and
> ld86 things any more since lilo was the last package on our systems that
> used them.
> 
> However, it would probably be a good idea to modify the kernel loader to
> lock out interrupts and explicitly set up the stack in its assembly startup
> code to insure that the stack is located correctly above the code in the
> same segment, rather than relying on the boot loader to do the right thing. 
> The existing setup code already insures that the other segment registers are
> equal but omits the stack segment register.  Also, because lilo (and
> others?) loads the data/code segment at 0X90000, the stack pointer would
> have to be set no higher than 0XA000 to avoid potential overwrites of the
> EBDA.  But I believe from my look at the code that the data/code sits below
> 0X8000 in the segment, so this should be fine.
> 
> If others think this is a good thing, I will test and submit a patch.

I think this is a good thing ;)

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