linux-kernel - Re: Hibernation Redesign

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Message-ID: <46940F45.3090005@goop.org>
Date:	Tue, 10 Jul 2007 15:59:17 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jeremy Maitin-Shepard <jbms@....edu>
CC:	Al Boldi <a1426z@...ab.com>, Pavel Machek <pavel@....cz>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Hibernation Redesign

Jeremy Maitin-Shepard wrote:
> I suppose that would be an interesting thing to look into.  Another
> possible approach for having the kernel run in non-contiguous memory is
> to specify a memmap exactly to the kernel on the command-line, as I
> believe is done for the crashdump kernels currently. 

That sounds very fragile.  It would be better to extend the bootparams 
to contain that information.

> I recall reading, though, that even with the relocatable
> kernel support, there are still significant alignment requirements for
> loading the kernel.  In particular, I seem to recall that it is
> necessary to load an x86 kernel at maybe a 16MB boundary, and on other
> platforms the alignment requirements may be even more restrictive.

2MB for x86, I think.  But that's not really an issue if you use a 
P(seudo-physical) to M(achine) mapping, since you can choose any 
arrangement you like for the kernel.  The only restriction is that you 
can't use large pages any more, but I don't think that's an issue for a 
dump/hibernation kernel.

>   In
> addition, I recall that the Linux boot procedure on x86 and on some
> other platforms necessarily uses certain low-address memory, like the
> first 640K, which must be backed up regardless.
>   

Well, the traditional framebuffer/ISA space between 640k and 1M probably 
needs to be identity mapped, but I don't think there's anything in there 
which specifically needs to be save/restored (except framebuffer 
contents, maybe?).

> For these reasons, it seems that it would be easiest to simply backup
> the first e.g. 16 or 64 MB of memory, and not have to worry about
> loading the kernel at a non-standard address and specifying a
> complicated exact memmap.  Someone might prove me wrong, though.
>   

Yes, I suppose.  You're certain the old kernel's devices are completely 
quiescent at that point?

    J

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