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

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Date:	Tue, 20 Nov 2012 16:28:18 -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 04:20 PM, Yinghai Lu wrote:
> 
> So will have
>   Bit 1 (read): LOADED_ABOVE_4G
>   - If 1, code, boot_param, cmdline, ramdisk could be loaded above 4G,
> 
> will update the patches accordingly.
> 

No, read the below.

>>
>> 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.
> 
> yes. try to make it ready for future is fun too.
> 
> are you going to have pointer for ext_header ?
> 

No, it doesn't help us much.

Overall, we need to keep growth of struct setup_header under some
control.  The highest usable offset is 0x280.

	-hpa

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