Re: [PATCH] kexec based hibernation: a prototype of kexec multi-stage load

["Huang, Ying" <ying.huang@intel.com>]

On Thu, 2008-05-15 at 19:25 -0700, Eric W. Biederman wrote:
> "Huang, Ying" <ying.huang@intel.com> writes:
> 
> > On Thu, 2008-05-15 at 11:39 -0700, Eric W. Biederman wrote:
> > [...]
> >> 2) After we figure out our address read the stack pointer from
> >>    a fixed location and simply set it.  (This is my preference)
> >
> > Just for confirmation (My English is poor).
> >
> > Do you mean that kernel A just read the stack top as re-entry point,
> > regardless of whether it is return address or argument 1?
> 
> What I was thinking was:
> 
> In kernel A()
> 
> relocate_new_kernel:
> 
>         ...
> 
>         call	*%eax
> 
> kexec_jump_back_entry:
>         /* This code should be PIC so figure out where we are */
>         call	1f
> 1:
>         popl	%edi
>         subl	$(1b - relocate_kernel), %edi
> 
>         /* Setup a safe stack */
>         leal    PAGE_SIZE(%edi), %esp
>         ...
> 
> 
> Then in purgatory we can read the address of kexec_jump_back_entry
> by examining 0(%esp) and export it in whatever fashion is sane.
> 
> However we reach kexec_jump_back_entry we should be fine.

I think it is reasonable to enable jumping back and forth more than one
time. So the following should be possible:

1. Jump from A to B (actually jump to purgatory, trigger the boot of B)
2. Jump from B to A
3. Jump from A to B again (jump to the kexec_jump_back_entry of B)
4. Jump from B to A
...

So it should be possible to get the re-entry point of kernel B in
kexec_jump_back_entry of kernel A too. So I think in
kexec_jump_back_entry, the caller's stack should be checked to get
re-entry point of peer. And the stack state is different depend on where
come from, from relocate_new_kernel() or return.

Best Regards,
Huang Ying