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

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, May 16, 2008 at 12:52:48PM +0800, Huang, Ying wrote:
> 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.

That's a good point. Remembering the actual return points in purgatory
will require purgatory to be saved along with core file.

I think, purgatory is a good infrastructure for transitions between the
kernels but at the same time, here it is a matter of just making a "call"
and then inspecting the stack in kexec_jump_back_entry. IMHO, we can keep it
simple and not involving purgatory in later transitions.

Thanks
Vivek