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

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Date:	Wed, 12 Dec 2007 14:37:41 -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:
> On Wed, Dec 12, 2007 at 11:05:45AM -0800, H. Peter Anvin wrote:
>> 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).
> 
> Ah, so the structure has changed over the years, making this not so much
> a "firmware" field as originally thought :)

The structure has been *amended* to over the years.  That's why the 
format is so awkward...

>>>> 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.
> 
> Can't kexec get this from within the kernel itself when it is running?
> It doesn't need to get this from userspace, does it?

It probably could, but it probably needs to modify some fields.

>>> 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.
> 
> I just don't want kernel structures exported in binary fashions in
> sysfs.  If there are specific portions that kexec currently can't figure
> out, why not just export those fields?
> 
> And, by exporting the different fields (yeah, lots of files, no big
> deal), you can handle the change in the structure over time much easier
> than trying to "know" the layout of the binary blob.

But if you kexec an older kernel from a newer kernel, the potential 
additional information will be lost for a subsequent uprevision kexec, 
for example.

	-hpa

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