linux-kernel - Re: [PATCH v2 03/13] x86: Add macro for 64bit entry startup

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Date:	Tue, 20 Nov 2012 13:12:26 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v2 03/13] x86: Add macro for 64bit entry startup_64

On 11/20/2012 12:55 PM, Yinghai Lu wrote:
> On Tue, Nov 20, 2012 at 12:44 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> I don't see any point in three flags here.  Let's stick to one flag; then it fits in the existing boot_flags field.
>
> the magic AA55
>
> boot/header.S:boot_flag:        .word 0xAA55
>
> if change that to other value, would it break existing boot loader?
> aka old boot loader could boot the new bzImage anymore.
>

Sorry, I meant loadflags.

This is a read flag and so should be low, bit 1 presumably.

On the other hand, the read flags in loadflags has not been modified for 
a very long time, and there is a serious risk that some broken 
bootloader might be doing a full byte comparison.

relocatable_kernel really should have been a flag, but it is now defined 
as a comparison with zero.  As such, and in an effort to minimize the 
growth of struct setup_header (it is limited to little over 128 bytes 
long) I suggest we redefine the 16-byte field at offset 0x236 as a new 
flags field.  We still only need one flag, though.

Backwards compatibility is so much fun.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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