[RFC PATCH v3 00/20] x86/kexec: Add exception handling for relocate_kernel and further yak-shaving

[David Woodhouse]

Debugging kexec failures is painful, as anything going wrong in execution
of the critical relocate_kernel() function tends to just lead to a triple
fault. Thus leading to *weeks* of my life that I won't get back. Having
hacked something up for my own use, I figured I should share it...

Add a CONFIG_KEXEC_DEBUG option which sets up a trivial exception handler
in that environment, and outputs to the early_printk serial console if
configured. Currently only I/O-based 8250 serial ports are supported, but
that could be extended.

While we're here, clean the code up a little and fix some other problems.
Most notably, load a suitable GDT on the way back into the kernel after a
KEXEC_PRESERVE_CONTEXT invocation instead of trusting the called code to
do so. And explicitly map the control_code_page into the identmap used by
relocate_kernel() instead of depending on it being in the same superpage
as something else that gets mapped.

I should probably bring the i386 version into line with this, although
the lack of rip-based addressing makes all the PIC code a bit harder.

David Woodhouse (20):
      x86/kexec: Ensure control_code_page is mapped in kexec page tables
      x86/kexec: Restore GDT on return from preserve_context kexec
      x86/kexec: Clean up and document register use in relocate_kernel_64.S
      x86/kexec: Use named labels in swap_pages in relocate_kernel_64.S
      x86/kexec: Only swap pages for preserve_context mode
      x86/kexec: Allocate PGD for x86_64 transition page tables separately
      x86/kexec: Copy control page into place in machine_kexec_prepare()
      x86/kexec: Invoke copy of relocate_kernel() instead of the original
      x86/kexec: Move relocate_kernel to kernel .data section
      x86/kexec: Add data section to relocate_kernel
      x86/kexec: Drop page_list argument from relocate_kernel()
      x86/kexec: Eliminate writes through kernel mapping of relocate_kernel page
      x86/kexec: Clean up register usage in relocate_kernel()
      x86/kexec: Mark relocate_kernel page as ROX instead of RWX
      x86/kexec: Add CONFIG_KEXEC_DEBUG option
      x86/kexec: Debugging support: load a GDT
      x86/kexec: Debugging support: Load an IDT and basic exception entry points
      x86/kexec: Debugging support: Dump registers on exception
      x86/kexec: Add 8250 serial port output
      [DO NOT MERGE] x86/kexec: Add int3 in kexec path for testing

 arch/x86/Kconfig.debug               |   8 +
 arch/x86/include/asm/kexec.h         |  34 +++-
 arch/x86/include/asm/sections.h      |   1 +
 arch/x86/kernel/callthunks.c         |   6 +
 arch/x86/kernel/early_printk.c       |   6 +
 arch/x86/kernel/machine_kexec_64.c   | 127 ++++++++----
 arch/x86/kernel/relocate_kernel_64.S | 385 +++++++++++++++++++++++++++--------
 arch/x86/kernel/vmlinux.lds.S        |  16 +-