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

Huang, Ying
2007-07-15T09:31:17Z

On Sat, 2007-07-14 at 21:16 +0200, Rafael J. Wysocki wrote:
> > The devices should be quiesced and the state of devices should be saved
> > in kexec_jump, before relocate_kernel is called. This needs the
> > implementation of device hibernating as you mentioned before.
>
> Hmm, at which point devices are normally shut down when kexec is used?

I think putting devices in quiescent state (not in low power state) is
sufficient for booting a new kernel with kexec, is it? According to my
experiment, the new kernel can be booted with kexec if the .suspend
method the drivers is called before kexec (given CONFIG_ACPI is not
selected).

Do we need a device quiesce/save + device shutdown for kexeced kernel to
work? I don't think so.

> > > > 4. In relocate_kernel, 0~16M is backupped firstly, then the
> > > > hibernating kernel and initramfs is copied to 0~16M, after that,
> > > > the hibernating kernel is booted.
> > > > 5. In hibernating kernel, the memory of normal kernel (it is in
> > > > 16M~512M) is saved into a hibernation image through /dev/mem
> > > > and ELF header.
> > >
> > > I don't think it can be _that_ simple:
> > > (a) what about processes' memory
> > > (b) what about areas that shouldn't be saved?
> >
> > The mem_map (struct page[]) of every zone of hibernated kernel is
> > checked. Necessary pages are saved, like memory snapshot of software
> > suspend, but in user space.
>
> Well, it's not enough to check that, sorry. That's why we have
> register_nosave_region().

After some investigation, I found the usage of "nosave" is as follow on
i386:

1. __nosavedata
used only for global variable in_suspend and swsusp_pg_dir
2. PG_nosave page flags
used for snapshot itself

Both are not necessary for kexec based hibernation. Because the image
are written from a different kernel, the memory of hibernating kernel
will not be saved, they can be used freely during image writing/reading.

On x86_64, there is another usage of nosave during processing E820
memory map. But I don't know why the memory region other than E820_RAM
are marked as nosave. I think only the memory region of type E820_RAM
will be thought of normal memory, others will be thought as reserved. Is
it sufficient just to check whether the page is reserved?

Best Regards,
Huang Ying