linux-kernel - Re: About support XZ-compressed kernel on x86

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Date:	Fri, 12 Feb 2016 23:41:41 +0800
From:	Baoquan He <bhe@...hat.com>
To:	lasse.collin@...aani.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, alain@...ff.lu,
	albin.tonnerre@...e-electrons.com, phillip@...gher.demon.co.uk,
	akpm@...ux-foundation.org, keescook@...omium.org, bp@...en8.de,
	vgoyal@...hat.com
Subject: Re: About support XZ-compressed kernel on x86

On 02/12/16 at 11:34pm, Baoquan He wrote:
> Hi Lasse,
> 
> I am checking a thread related to kaslr and setup_data issue on x86:
> https://lkml.org/lkml/2015/7/7/963
> 
> Now I have a question about the commit from you:
> 
> commit 303148045aac34b70db722a54e5ad94a3a6625c6
> Author: Lasse Collin <lasse.collin@...aani.org>
> Date:   Wed Jan 12 17:01:24 2011 -0800
> 
>     x86: support XZ-compressed kernel
> 
> 
> In this commit for adding support of XZ-compressed kernel on x86, you
> add extra 32K to the extract_offset. In commit log you said this is
> because "The XZ decompressor needs around 30 KiB of heap, so the heap
> size is increased to 32 KiB on both x86-32 and x86-64." With my
> understanding decompression is done in decompression stage and it uses
> boot_heap in arch/x86/boot/compressed/head_64.S, and boot_heap is
> assigned to free_mem_ptr which is used for decompression heap malloc.
> During this decompressio stage it's still in copied ZO space, why did
> you add extra 32K space to extract_offset?  If you want to increase the
> decompression heap space shouldn't you decrease the extract_offset? Do I
> misunderstand anything or miss things?
> 
> Please help answer, I really don't understand this.

Lasse, please help have a look when it's convenient to you, many thanks.

> 
> Thanks
> Baoquan