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SubjectRe: Hibernation considerations
On Tue, 17 Jul 2007, Al Boldi wrote:

> wrote:
>> On Mon, 16 Jul 2007, Al Boldi wrote:
>>> We have to go through ACPI, for wakeup functions to succeed. A simple
>>> power-off won't do.
>> the kexec switch being posted requires ACPI be disabled, so it's clearly
>> possible to switch kernels and initialize devices without ACPI
> It's a given that kexec works in the absence of ACPI; what we have to handle
> is the ACPI states across kernel invocations, to ensure wakeup functions
> succeed. If you don't need this, then just power off.
>>>> suspend-to-disk-and-ram could be implemented as three
>>>> seperate steps
>>>> 1. suspend-to-disk
>>>> 2. resume-from-disk
>>>> 3. suspend-to-ram
>>>> followed by either
>>>> 4. resume-from-ram
>>>> or
>>>> 4. battery dies and loptop powers off completely
>>>> 5. power-on boot.
>>>> 6. resume-from-disk
>>>> all that you need to do is to make sure that the system doesn't run
>>>> anything that would affect permanent media or the outside world between
>>>> steps #2 and #3
>>> Exactly, which is why your scheme would break down on #3, and that's why
>>> you need to call S3 from within the kexec'd hibernation kernel after
>>> saving the hibernation image.
>> when a kexec is called, how does the kernel know what to execute?
>> something needs to tell it what to do, and I think that something is
>> either something in the kexec image, or it's something passed as a
>> parameter to that image.
>> all that would be needed to do #3 safely is to have the kernel that you
>> restarted on #2 do a suspend-to-ram before it does anything else.
> If you mean by kernel 'the normal kernel', then this won't work, because it
> would imply a change of state after saving its image.

yes, it would change the state, but if it only changes the state in ways
that aren't visable to the outside world why would it matter?

if power dies you restore from the disk image (useing the non ACPI
approach), and the changes that you make are just lost

> If you mean by kernel 'the kexec'd hibernation kernel', then you wouldn't
> need to do #2, but rather do #3 right after dumping the image in #1.
> [...insert from another post...]
>>> BTW, it would be really helpful if people would actually try the kexec
>>> hibernation patches, as this may yield a much more constructive
>>> discussion.
>> I would love to, but so far I don't see the nessasary pieces
>> once I kexec to the new kernel, how can it find out what pages of memory
>> (and swap) need to be saved?
> No need to save the swap, all you need to do is to dump /dev/oldmem onto
> storage, and if that dump image is compatible with swsusp, then a normal
> kernel should be able to resume from this image via /dev/snapshot.

Rafael is saying that there's more involved, you can't just dump
/dev/oldmem, you have to avoid specific pages.

as for swap, saving that may be required, depending on how clean you want
toleave the box for other OS's in the meantime.

David Lang
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