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

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Message-ID: <50AABD15.1050809@zytor.com>
Date:	Mon, 19 Nov 2012 15:13:25 -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 11/19/2012 02:53 PM, Yinghai Lu wrote:
>>
>> We can't change the 0x200 value, ever, because there are already things
>> that use the 64-bit entry point.  As such, these macros and the bzImage
>> fields are pointless; let's not bother and instead document the 0x200 as
>> the permanently fixed address of the 64-bit entry point.
> 
> ok,
> 
> any other field, in header struct field that we can use to tell
> bzImage could be used that
> 0x200 directly?
> 
> hardware_subarch?
> 

There isn't one... this dates back all the way to the original x86-64
kernels.

Are you asking if we can tell this is a 64-bit kernel (as opposed to a
32-bit kernel, which obviously doesn't have a 64-bit entry point)?
Unfortunately there isn't an intentional one that I know of.  There
might be an accidental such indicator, but we'd have to go back to look
at 8+ years of kernels.  We can't even rely on a jmp instruction at the
address...

	-hpa

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