linux-kernel - Re: RFC: x86: relocatable kernel changes (revised spec)

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Message-ID: <4A084C36.3010808@zytor.com>
Date:	Mon, 11 May 2009 09:03:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"hbabu@...ibm.com" <hbabu@...ibm.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"Huang\, Ying" <ying.huang@...el.com>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"sam@...nborg.org" <sam@...nborg.org>
Subject: Re: RFC: x86: relocatable kernel changes (revised spec)

Eric W. Biederman wrote:
> 
>> +Field name:	init_size
>> +Type:		read
>> +Offset/size:	0x25c/4
>> +
>> +  This field indicates the amount of linear contiguous memory starting
>> +  at the kernel load address (rounded up to kernel_alignment) that the
>> +  kernel needs before it is capable of examining its memory map.  This
>> +  is not the same thing as the total amount of memory the kernel needs
>> +  to boot, but it can be used by a relocating boot loader to help
>> +  select a safe load address for the kernel.
> 
> This wording is a bit unclear.
> 
> Can we finally say that it is safe to put the initrd immediately after
> the kernel?
> 

I *believe* we can, as the brk limit checking should catch overruns.
The only question is whether or not there will me memory allocated off
the memory map before the initrd is reserved; I *think* the answer is no
but I haven't done the audit.

> The rounding up part of that comment is unclear.

The rounding up was to reflect the automatic moving upwards from the
load address to the next kernel_alignment datum.

> Peter did your implementation of init_size take into account the maximum expansion
> during decompression?  At a quick glance at your previous patches I couldn't
> tell.  I know were in that direction with zoffset.h and voffset.h but I don't
> recognize the formula for where I put the pic decompressor in your calculation
> of this.

It does take it into account.  The pic decompressor is located at
(ZO_)z_extract_offset; the actual formula moved into mkpiggy.c.

I have regenerated the tip:x86/kbuild-phys branch to be only cleanups
(with the intent of putting the policy changes cleanly on top), and much
better structured.  I hadn't originally expected this to turn into so
much of a cleanup effort.

http://git.kernel.org/?p=linux/kernel/git/x86/linux-2.6-tip.git;a=shortlog;h=x86/kbuild-phys

This checkin, in particular, should answer that question, I believe:

http://git.kernel.org/?p=linux/kernel/git/x86/linux-2.6-tip.git;a=commitdiff;h=02a884c0fe7ec8459d00d34b7d4101af21fc4a86

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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