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 at 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
