linux-kernel - Re: [PATCH -mm -v2] x86 boot : export boot

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Date:	Wed, 12 Dec 2007 11:05:45 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Greg KH <greg@...ah.com>
CC:	"Huang, Ying" <ying.huang@...el.com>, akpm@...ux-foundation.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Andi Kleen <ak@...e.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm -v2] x86 boot : export boot_params via sysfs

Greg KH wrote:
>> This is a binary structure defined by protocol;
> 
> What protocol?  Is this a "standard" documented somewhere?

Yes, see Documentation/i386/* (although some of it is documented by 
reference to include/asm-x86/boot_params.h).

>> in that way it's not significantly different from something passed
>> from the firmware (in fact, it might very well *be* passed from the
>> firmware.)  We have in the past found platform bugs by looking at the
>> contents of the whole structure, e.g.  to find that part of it has
>> been inappropriately clobbered.
> 
> For debugging things, then just export it through debugfs.

Fair enough, however...

>> It is also in the form needed by e.g. kexec to operate.
> 
> Does kexec need this today to work properly?  Or is this something new?

I believe kexec currently tries to reconstitute it from what data is 
available to it.  This is incomplete, though, and has been flagged as a 
problem for kexec.

> What userspace program is going to know the exact data format of this
> blob, and where is it going to know that format from?  The kernel header
> files in sanitized form?  Or something else?

It can pick it up from <asm/boot_params.h> (which is now 
userspace-safe); or it can decode it itself.  Programs like kexec can 
pass through most of the data without examining it, this is the main 
reason for having it as a blob.

	-hpa
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