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

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Date:	Mon, 19 Nov 2012 15:46:46 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Mon, Nov 19, 2012 at 3:13 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 11/19/2012 02:53 PM, Yinghai Lu wrote:
>>
>> 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...

So we could add one field to tell that bzImage could be used with 64bit?

current in this patchset, I added

0268/4  2.12+   ext_ramdisk_image ramdisk_image 32 bits
026C/4  2.12+   ext_ramdisk_size ramdisk_size high 32 bits
0270/4  2.12+   code64_start_offset 64bit start offset for bzImage
0274/4  2.12+   ext_cmd_line_ptr cmd_line_ptr high 32 bits

so you don't like code64_start_offset.

how about other three?

can we use bits 31 of hardware_subarch to tell it is bzImage for x86_64?

Thanks

Yinghai
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