linux-kernel - Re: why the decompressed procedure move kernel from address 0x100000(1M) to 0x1000000(16M) +x

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Date:	Sat, 02 Jun 2012 17:48:04 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	hacklu <embedway.linux@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: why the decompressed procedure move kernel from address 0x100000(1M) to 0x1000000(16M) +x

hacklu <embedway.linux@...il.com> writes:

> hi all,
> recently, I got some puzzle when I read source code of the system boot. I need
> some help.
>
> at the end of src/arch/x86/boot/header.S,  kernel jump to 0x100000(where is the
> src/arch/x86/boot/compressed/head_32.S).
> in __this__ head_32.S, I found the kernel is move to 0x1000000(mostly is to
> here) +x. the x distance is used for decompressed buf.  must leave some distance
> for decompressing without overlap.
>
> after the move, kernel is decompressed at 0x1000000(16m). and jump to it.
>
> so why not decompressed kernel at 0x100000(1M) to 0x1000000(16m) directly
> without moving?
>
> is the move necessary?

The move is nececcessary if we are doing the decompression in place.
Without a move it is hard to tell if there are going to be overlapping
address problems.  The move is cheap so there is no apparent reason
to optimize it away.

Eric
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