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

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

On Thu, 2008-05-15 at 19:55 -0700, Eric W. Biederman wrote:
> "Huang, Ying" <ying.huang@intel.com> writes:
> 
> > The disadvantage of this solution is that kernel B must know it is
> > original kernel (A) or kexeced kernel (B). Different code should be used
> > by kernel A and kernel B. And after jump from A to B, jump from B to A,
> > when jump from A to B again, kernel A must use different code from the
> > first time.
> 
> I don't know what the case is for keeping two kernels in memory and switching
> between them.

This can be used to save the memory image of kernel B and accelerate the
hibernation. The real boot of kernel B is only needed first time.

> I suspect a small piece of trampoline code between the two kernels could
> handle the case. (i.e. purgatory pays attention).
> 
> That is a fundamental aspect of the design.  A general purpose infrastructure
> with trampoline code to adapt it to whatever situation comes up.

It is possible to use purgatory to deal with this problem.

Jump from kernel A to kernel B
    Jump to entry of purgatory (purgatory_entry)
    purgatory save the return address (kexec_jump_back_entry_A)
    Purgatory set kexec_jump_back_entry for kernel B to a code
        segment in purgatory, say kexec_jump_back_entry_A_for_B
    Purgatory jump to entry point of kernel B
Jump from kernel B to kernel A
    Jump to purgatory (kexec_jump_back_entry_A_for_B)
    Purgatory save the return address (kexec_jump_back_entry_B)
    Purgatory return to kernel A (kexec_jump_back_entry_A)
Jump from kernel A to kernel B again
    Jump to entry of purgatory (purgatory_entry)
    Purgatory save the return address (kexec_jump_back_entry_A)
    Purgatory jump to kexec_jump_back_entry_B

The disadvantage of this solution is that some information is saved in
purgatory (kexec_jump_back_entry_A, kexec_jump_back_entry_B). So,
purgatory must be saved too when save the memory image of kernel A or
kernel B. Purgatory can be seen as a part of kernel B. But it is a
little tricky to think it as a part of kernel A too.

Best Regards,
Huang Ying