Re: [PATCH -mm] kexec jump -v9

[Huang, Ying]

On Tue, 2008-03-11 at 17:10 -0400, Vivek Goyal wrote: 
> On Thu, Mar 06, 2008 at 11:13:08AM +0800, Huang, Ying wrote:
> > This is a minimal patch with only the essential features. All
> > additional features are split out and can be discussed later. I think
> > it may be easier to get consensus on this minimal patch.
> > 
> 
> Hi Huang,
> 
> This patchset is slowly getting better. True that first we need to come
> up with minimal infrastructure patch and then think of building more
> functionality on top of it.
> 
> 
> > This patch provides an enhancement to kexec/kdump. It implements
> > the following features:
> > 
> > - Jumping between the original kernel and the kexeced kernel.
> > 
> > - Backup/restore memory used by both the original kernel and the
> >   kexeced kernel.
> > 
> > - Save/restore CPU and devices state before after kexec.
> > 
> > 
> > The features of this patch can be used for as follow:
> > 
> > - A simple hibernation implementation without ACPI support. You can
> >   kexec a hibernating kernel, save the memory image of original system
> >   and shutdown the system. When resuming, you restore the memory image
> >   of original system via ordinary kexec load then jump back.
> > 
> 
> The main usage of this functionality is for hibernation. I am not sure
> what has been the conclusion of previous discussions.
> 
> Rafael/Pavel, does the approach of doing hibernation using a separate
> kernel holds promise?
> 
> [..]
> > Usage example of simple hibernation:
> > 
> > 1. Compile and install patched kernel with following options selected:
> > 
> > CONFIG_X86_32=y
> > CONFIG_RELOCATABLE=y
> > CONFIG_KEXEC=y
> > CONFIG_CRASH_DUMP=y
> > CONFIG_PM=y
> > 
> > 2. Build an initramfs image contains kexec-tool and makedumpfile, or
> >    download the pre-built initramfs image, called rootfs.gz in
> >    following text.
> > 
> > 3. Prepare a partition to save memory image of original kernel, called
> >    hibernating partition in following text.
> > 
> > 3. Boot kernel compiled in step 1 (kernel A).
> > 
> > 4. In the kernel A, load kernel compiled in step 1 (kernel B) with
> >    /sbin/kexec. The shell command line can be as follow:
> > 
> >    /sbin/kexec --load-preserve-context /boot/bzImage --mem-min=0x100000
> >      --mem-max=0xffffff --initrd=rootfs.gz
> > 
> > 5. Boot the kernel B with following shell command line:
> > 
> >    /sbin/kexec -e
> > 
> > 6. The kernel B will boot as normal kexec. In kernel B the memory
> >    image of kernel A can be saved into hibernating partition as
> >    follow:
> > 
> >    jump_back_entry=`cat /proc/cmdline | tr ' ' '\n' | grep kexec_jump_back_entry | cut -d '='`
> >    echo $jump_back_entry > kexec_jump_back_entry
> >    cp /proc/vmcore dump.elf
> > 
> 
> Why not store the entry point in dump.elf itself, instead of storing it
> in a separate file?
> 
> I think this is more like a resumable core file. Something similar to
> functionality what qemu does for resuming an already booted kernel image.
> So we might have to introduce an ELF_NOTE to mark an image as resumable
> core. 

Yes. The entry point should be saved in dump.elf itself, this can be
done via a user-space tool such as "makedumpfile". Because
"makedumpfile" is also used to exclude free pages from disk image, it
needs a communication method between two kernels (to get backup pages
map or something like that from kernel A). We have talked about this
before.

- Your opinion is to communicate via the purgatory. (But I don't know
how to communicate between kernel A and purgatory).
- Eric's opinion is to communicate between the user space in kernel A
and user space in kernel B.
- My opinion is to communicate between two kernel directly.

I think as a minimal infrastructure patch, we can communicate minimal
information between user space of two kernels. When we have consensus on
this topic, we can use makedumpfile for both excluding free pages and
saving the entry point. Now, we can save the entry point in a separate
file or I can write a simple tool to do this.

> >    Then you can shutdown the machine as normal.
> > 
> > 7. Boot kernel compiled in step 1 (kernel C). Use the rootfs.gz as
> >    root file system.
> > 
> > 8. In kernel C, load the memory image of kernel A as follow:
> > 
> >    /sbin/kexec -l --args-none --entry=`cat kexec_jump_back_entry` dump.elf
> > 
> 
> How the memory segments of dump.elf loaded? Normal kexec way? Memory
> segments of dump.elf are first stored somewhere and then moved to
> destination at "kexec -e" time?

Yes. Exactly. But during kexec loading, if the source page is same as
destination page, we need just one page.

> Does this really work? If we have 4G RAM, what will be the size of
> dump.elf? And when we load it back for resuming, do we have sufficient
> memory left?

Yes. It really works. If we have 4G RAM, the size of dump.elf is 4G -
(memory area used by second kernel), in this example, it is 4G - 16M.
The loading kernel will live in 16M memory, and load dump.elf into all
other memory area.

> May be we can have a separate load flag (--load-resume-image) to mark
> that we are resuming an hibernated image and kexec does not have to
> prepare commandline, does not have to prepare zero page/setup page etc.

There is already similar flag in original kexec-tools implementation:
"--args-none". If it is specified, kexec-tools does not prepare command
line and zero page/setup page etc. I think we can just re-use this flag.
And If it is desired an alias is good for me too.

> I have thought through it again and try to put together some of the
> new kexec options we can introduce to make the whole thing work. I am 
> considering a simple case where a user boots the kernel A and then
> launches kernel B using "kexec --load-preseve-context". Now a user
> might save the hibernated image or might want to come back to A.
> 
> - kexec -l <kernel-image>
>         Normal kexec functionality. Boot a new kernel, without preserving
>         existing kernel's context.
> 
> - kexec --load-preserve-context <kernel-image>
>         Boot a new kernel while preserving existing kernel's context.
> 
>         Will be used for booting kernel B for the first time.
> 
> - kexec --load-resume-image <resumable-core>

In original kexec-tools, this can be done through:
kexec -l --args-none <resumable-core>

Do you need to define an alias for it?

>         Resumes an hibernated image. Load a ELF64 hibernated image.
> 
> 	Context of first kernel/boot-loader will not be preserved.
> 
> 	First kernel will not save cpu states. Will put devices into
> 	suspended state though so that these can be resumed by resumable
> 	core
> 
>         This option can be used by kboot or kernel C to resume an hibernated
> 	image.
> 
> - kexec --load-resume-entry <entry-point>

In current implementation, this can be done through:
kexec --load-jump-back-helper --entry <entry-point>.

I think the new name is good.

>         Image is already loaded. Just prepare the entry point so that one
>         can enter back to previous image. cpu states will be saved and devices
>         will be put to suspended states.
> 
>         will be used for A --> B and B ---> A transitions. Both A and B are
>         booted. This is just for switching back and forth between A and B.
> 
> - kexec -e
>         Transition into the new kernel
> 
> This patch looks in pretty decent shape. Once there is some sort of
> understanding that this approach is promising for hibernation and we
> have consensus on high level interface, then we can get into line by 
> line review of the patch set. 
> 

Best Regards,
Huang Ying
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