[PATCH v8 0/7] x86/kexec: Add exception handling for relocate_kernel

[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 trivial exception handler in the relocate_kernel environment which 
outputs to the early_printk serial console if configured. Currently only 
8250-compatible serial ports are supported, but that could be extended.

I had to hack up QEMU support for a PCI serial port which matches what
the existing early_printk code can drive, and the *real* 8250_pci driver
doesn't seem to cope with that setup at all, but whatever... the kexec
code now drives the same 32-bit stride which is all that earlyprintk
supports. We can always add more later, if anyone cares.

Someone who cares might want to bring the i386 version into line with
this, although the lack of rip-based addressing makes all the PIC code a
bit harder.

v8:
 • Fix UNRET objtool warning in exc_handler.
 • Clean up magic numbers in stack frame for exc_handler.
 • Fix i386 build error due to making the debug support unconditional.
 • The int3 is still a [DO NOT APPLY] hack for later, and I plan to deal
   with that with a userspace test case based on
   http://david.woodhou.se/loadret.c which will exercise kexec-jump at the
   same time.

v7: https://lore.kernel.org/kexec/20250312144257.2348250-1-dwmw2@xxxxxxxxxxxxx/
 • Drop CONFIG_KEXEC_DEBUG and make it all unconditional in order to
   "throw regressions back into the face of whoever manages to introduce
   them" (Ingo, https://lore.kernel.org/kexec/Z7rwA-qVauX7lY8G@xxxxxxxxx/)
 • Move IDT invalidation into relocate_kernel() itself.

v6: https://lore.kernel.org/kexec/20250115191423.587774-1-dwmw2@xxxxxxxxxxxxx/
 • Rebase onto already-merged fixes in tip/x86/boot.
 • Move CONFIG_KEXEC_DEBUG to generic kernel/Kconfig.kexec as Bartosz is
   working on an Arm64 version.

v5: https://lore.kernel.org/kexec/20241205153343.3275139-1-dwmw2@xxxxxxxxxxxxx/T/
 • Drop [RFC].
 • Drop _PAGE_NOPTISHADOW fix, which Ingo already took into tip/x86/urgent.
 • Add memory-mapped serial port support (32-bit MMIO spacing only).

v4 (RFC): https://lore.kernel.org/kexec/20241127190343.44916-1-dwmw2@xxxxxxxxxxxxx/T/
 • Add _PAGE_NOPTISHADOW fix for the identmap code.
 • Drop explicit map of control page, which was masking the identmap bug.

v3 (RFC): https://lore.kernel.org/kexec/20241125100815.2512-1-dwmw2@xxxxxxxxxxxxx/T/
 • Add CONFIG_KEXEC_DEBUG option and use earlyprintk config.
 • Allocate PGD separately from control page.
 • Explicitly map control page into identmap.

V2 (RFC): https://lore.kernel.org/kexec/20241122224715.171751-1-dwmw2@xxxxxxxxxxxxx/T/
 • Introduce linker script, start to clean up data access.

V1 (RFC): https://lore.kernel.org/kexec/20241103054019.3795299-1-dwmw2@xxxxxxxxxxxxx/T/
 • Initial proof-of-concept hacks.

David Woodhouse (7):
      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
      x86/kexec: Add 8250 MMIO serial port output
      x86/kexec: Invalidate GDT/IDT from relocate_kernel() instead of earlier
      [DO NOT MERGE] x86/kexec: Add int3 in kexec path for testing
      [DO NOT MERGE] x86/kexec: Add CFI type information to relocate_kernel()

 arch/x86/include/asm/kexec.h         |   7 +
 arch/x86/kernel/early_printk.c       |   9 ++
 arch/x86/kernel/machine_kexec_64.c   |  50 +++++--
 arch/x86/kernel/relocate_kernel_64.S | 248 ++++++++++++++++++++++++++++++++++-
 4 files changed, 302 insertions(+), 12 deletions(-)