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    SubjectRe: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation
    On Saturday, 14 July 2007 07:48, Huang, Ying wrote:
    > On Fri, 2007-07-13 at 10:43 -0600, Eric W. Biederman wrote:
    > > > Why a assembly stub is necessary? Is it not sufficient that just
    > > > continue to complete a normal boot (hot add the reset of memory) or load
    > > > the hibernated kernel (hibernated image) and jump to it?
    > >
    > > I was thinking the assembly stub would be the small piece that jumps
    > > to loaded hibernated kernel. Quite possibly we could just get away
    > > with providing no memory and just an entry point to kexec but it
    > > makes sense to me to plan on running a couple of instructions.
    > Oh, I got it. In my patch, there is such assembly stub in
    > arch/i386/kernel/kexec_jump.S. I think it is needed to restore basic CPU
    > state and accommodate some position independent restore code (such as
    > memory restore code).
    > > Actually the way the kexec infrastructure it might be reasonable to
    > > just use sys_kexec_load to load the entire hibernated image. Except
    > > for the fact that sys_kexec_load requires the source pages to be
    > > in the processes memory image the code shouldn't have the 50% of
    > > memory limitation already.
    > >
    > > If we can get that going we don't even need to restrict the first
    > > kernels memory. So it might just require teaching sys_kexec_load
    > > how to steal process pages. Anyway something to think about.
    > As for memory backupping and restoring during hibernating and resuming,
    > I think a possible picture can be as follow:
    > Memory:
    > Total memory: 512M
    > Memory used by hibernating/resuming kernel: 0~16M
    > Hibernating process:
    > 1. Normal kernel running
    > 2. Hibernating is triggered, sys_kexec_load is used to load
    > hibernating kernel and initramfs into memory. Then
    > sys_reboot(LINUX_REBOOT_CMD_KSPAWN) is invoked.
    > 3. In sys_reboot, kexec_jump is called to save device/CPU state,
    > then relocate_kernel is called. kexec_jump and relocate_kernel
    > reside in individual page in 16M~512M.


    What's going to happen to devices at this point?

    > 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?

    > Resume process:
    > 1. Resuming kernel is booted as a normal kernel, but the memory is
    > restricted to 0~16M.
    > 2. Checking whether there is a effective hibernation image. If
    > there isn't, the memory of 16M~512M is hot added, and the normal
    > boot up process continues; If there is, a resuming process is
    > triggered.
    > 3. sys_kexec_load is used to restore the memory state of hibernated
    > kernel. The sys_kexec_load works in crashdump way, that is, the
    > hibernation image is copied to destination location in 16M~512M
    > in sys_kexec_load instead of relocate_kernel. There is no half
    > of memory size restriction.
    > 4. sys_reboot is called to trigger jumping back, which will jump back
    > to kexec_jump of hibernated kernel.
    > 5. In kexec_jump of hibernated kernel, the memory of 0~16M is copied
    > back from the backup area in 16M~512M. The memory state of
    > hibernated kernel is restored totally. The CPU and device state
    > can be restored after that.

    Well, I don't know why this needs to be that complicated. We already have
    code in the mainline that's able to load a large hibernation image into memory
    and jump to the kernel being restored. And it has _no_ 50% of RAM limitation,
    this is the _saving_ part of the current code that this limitation comes from.


    "Premature optimization is the root of all evil." - Donald Knuth
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