[PATCH v3 1/3] LoongArch: kexec: add KHO support for FDT-based

[George Guo]

Hi Huacai, Pratyush,

Thanks for the direction.  You were both right that the v3 approach was
the wrong shape, and I have dropped it.

Let me restate the constraint and report what I did.

> But LoongArch is different, the efistub doesn't create a FDT, instead
> it passes the EFI system table and command line to the core kernel
> directly.

So LoongArch has no boot FDT, and the arm64 /chosen path does not apply.
My v3 synthesized an FDT and rewrote the DEVICE_TREE_GUID entry only to
reuse the generic reader.  That was the hacky part, and it is gone.

I followed the x86 model instead.  x86 has no boot FDT either; it carries
the KHO pointer in setup_data (struct kho_data) and calls kho_populate()
directly.  LoongArch has no setup_data, but it does pass the EFI system
table.  So I use the EFI configuration table as the channel:

- The first kernel builds a small handover struct (the KHO FDT and
  scratch addresses), adds a dedicated GUID entry pointing to it, and
  loads both as kexec segments.  machine_kexec() switches the system
  table to the extended table before jumping.
- The next kernel scans the configuration table for that GUID, reads the
  struct, and calls kho_populate() directly.

There is no synthesized FDT, no DEVICE_TREE_GUID rewrite, and nothing on
the command line.  I did not go back to the v1 command line because that
exposes the addresses to unprivileged userspace through /proc/cmdline.

I tested this on real LoongArch hardware (ACPI/UEFI) with the live update
two-stage kexec test and the test passes:

$ sudo ./luo_kexec_simple --stage 1
# [STAGE 1] Starting pre-kexec setup...
# [STAGE 1] Creating state file for next stage (2)...
# [STAGE 1] Creating session 'test-session' and preserving memfd...
# [STAGE 1] Forking persistent child to hold sessions...
# [STAGE 1] Child PID: 1242. Resources are pinned.
# [STAGE 1] You may now perform kexec reboot.
$ sudo kexec -l /boot/vmlinuz-7.1.0-rc6 --initrd=/boot/initramfs-7.1.0-rc6.img --reuse-cmdline
$ sudo kexec -e

$ sudo ./luo_kexec_simple --stage 2
# [STAGE 2] Starting post-kexec verification...
# [STAGE 2] Retrieving session 'test-session'...
# [STAGE 2] Restoring and verifying memfd (token 0x1a)...
# [STAGE 2] Test data verified successfully.
# [STAGE 2] Finalizing test session...
# [STAGE 2] Finalizing state session...
#
--- SIMPLE KEXEC TEST PASSED ---
$ sudo dmesg | grep kexec
[    0.000000] [      T0] KHO: found kexec handover data.

I will send this as v4, folding in Mike's two selftest comments.  If you
see a problem with the EFI configuration table approach, please tell me
before I post.

Thanks,
George