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

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Date:	Thu, 12 Jul 2007 20:12:33 -0700 (PDT)
From:	david@...g.hm
To:	Jeremy Maitin-Shepard <jbms@....edu>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Huang, Ying" <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org
Subject: Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

On Thu, 12 Jul 2007, Jeremy Maitin-Shepard wrote:

> "Rafael J. Wysocki" <rjw@...k.pl> writes:
>
> [snip]
>
>> There's more to it, though.  If devices are suspended, the hibernation kernel
>> will have to resume them (using platform, like ACPI, callbacks in the process)
>> instead and that will get complicated.
>
>> It's better if devices are quiesced, or even shut down, before we call the
>> hibernation kernel.
>
> I agree that they definitely should not be put into a low power mode, as
> that has nothing to do with hibernation.
>
> Ideally, the following would be done:
>
> All of the hardware that won't be needed by the "save image" kernel will
> be shut down.  The normal driver shut down calls may not be suitable,
> however, because although the same thing should be done to the hardware,
> the device shouldn't be "unregistered", since unlike in the actual
> shutdown case, the same device will need to brought back up again on
> resume, and it will need to have the same device id and such (and
> userspace probably shouldn't see the device going away).
>
> Any devices that will be needed by the "save image" kernel could also
> safely be shutdown as with the unneeded devices, but it would be more
> efficient to simply quiesce it.  Since this would be an additional
> complication, initially probably all of the hardware should be shut
> down, rather than quiesced.
>
> The reason that I think it is useful to actually shut down the devices,
> rather than merely leaving some unneeded devices quiesced, is that it
> would be useful to be able to build the "save image" kernel without
> support for unneeded devices.  In order to support "suspend to ram"
> instead of shutting down after saving the image to disk, the hibernate
> kernel needs to be able to send devices into a low power state.  My
> impression is that if there are devices it does not know about (i.e. the
> unneeded devices), but which are left quiesced but powered on, this
> would be a problem for suspend to ram, although not knowing much about
> how suspend to ram actually works, I could be mistaken.  (Maybe it is
> possible through ACPI or standard bus interfaces to shut down all of the
> devices without really knowing anything about them.)

I don't think that anyone is talking about useing kexec for 
suspend-to-ram, only for suspend-to-disk (hibernate)

>>>>> 3. Support the in-place kexec? The relocatable kernel is not necessary
>>>>> if this can be implemented.
>>>>> 4. Image writing/reading. (Only user space application is needed).
>>>>
>>>> And a kernel interface for that application.
>>>
>>> I do't understand this statement, this application is just useing the
>>> standard kernel interfaces (block devices to read/write to disk, network
>>> devices to read/write to a server, etc). no new interfaces needed.
>
>> Yes, but it will have to know _what_ to save, no?
>
> I agree that a kernel interface would be important; something like
> /dev/snapshot that can be read by the "save image" kernel, and written
> to by the "restore image" kernel.  Note that similarly, kdump provides a
> kernel interface to an ELF image of the old kernel.

I thought that the idea was to save the entire contents of ram so that 
caches, etc remain populated.

having the system kernel free up ram and then making a sg list of what 
memory needs to be backed up would be a nice enhancement, but let's let 
that remain a future enhancement until everyone agrees that the basic 
approach works.

David Lang
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