linux-kernel - Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

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Date:	Thu, 12 Jul 2007 12:09:31 -0700 (PDT)
From:	david@...g.hm
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	"Huang, Ying" <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Jeremy Maitin-Shepard <jbms@....edu>,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	Kexec Mailing List <kexec@...ts.infradead.org>
Subject: Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

On Thu, 12 Jul 2007, Eric W. Biederman wrote:

>> 2. Do not reserve memory for kexec kernel. That is, backup needed memory
>> before kexec and restore them after kexec.
>> 3. Support the in-place kexec? The relocatable kernel is not necessary
>> if this can be implemented.
>
> It sounds like what you really want is the normal kexec path enhanced
> so that you can return to the kernel you started with.
>
> The normal kexec path already knows how to do the memory shuffle so
> it can do on demand memory allocation.  That code just needs to
> enhanced slightly so that you allocate an extra page, setup an inverse
> scatter gather list for restoring the pages, and teach relocate_kernel.S
> to preserve it's destination pages by using the inverse scatter gather
> list.
>
> The normal kexec path already calls device_shutdown and the like to
> stop devices from running.  Although again that code path is not
> prepared to restore the devices.

we shouldn't need a restore code path if the new kernel re-detects 
everything. if kexec already shuts down all the devices we may not need to 
implement anything new here (although there may be room for future 
performance optimization)

> ...
>
> For prototyping I would:
> - reserve a chunk of memory (possibly with the crashkernel= option)
>  and run a relocatable kernel out of it.
>
>  By using the normal kexec you can boot a relocatable restore kernel
>  in that reserved region. It is an extra step but it makes things
>  work today.
>
> - I would use the normal sys_kexec_load.
>
> - I would debug/tweak the user space and the code to reenter the
>  old kernel.  I.e. the device driver stop/start code.
>
>  Once it was basically working I would the update normal kexec
>  memory copy code in relocate.S to preserve the destination pages.

for prototyping there's no need to use the same kernel.

>> 4. Image writing/reading. (Only user space application is needed).
>
> And possibly a few fixes to /dev/mem.  This is pretty much the same
> process as generating a core dump so there should be some synergy with that.

what fixes are you thinking of?

you are makeing this sound very simple ;-)

David Lang
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