[PATCH v1 0/8] arm64: MMU enabled kexec relocation

[Pavel Tatashin]

Enable MMU during kexec relocation in order to improve reboot performance.

If kexec functionality is used for a fast system update, with a minimal
downtime, the relocation of kernel + initramfs takes a significant portion
of reboot.

The reason for slow relocation is because it is done without MMU, and thus
not benefiting from D-Cache.

Performance data
----------------
For this experiment, the size of kernel plus initramfs is small, only 25M.
If initramfs was larger, than the improvements would be greater, as time
spent in relocation is proportional to the size of relocation.

Previously:
kernel shutdown	0.022131328s
relocation	0.440510736s
kernel startup	0.294706768s

Relocation was taking: 58.2% of reboot time

Now:
kernel shutdown	0.032066576s
relocation	0.022158152s
kernel startup	0.296055880s

Now: Relocation takes 6.3% of reboot time

Total reboot is x2.16 times faster.

Previous approaches and discussions
-----------------------------------
https://lore.kernel.org/lkml/20190709182014.16052-1-pasha.tatashin@xxxxxxxxxx
reserve space for kexec to avoid relocation, involves changes to generic code
to optimize a problem that exists on arm64 only:

https://lore.kernel.org/lkml/20190716165641.6990-1-pasha.tatashin@xxxxxxxxxx
The first attempt to enable MMU, some bugs that prevented performance
improvement. The page tables unnecessary configured idmap for the whole
physical space.

https://lore.kernel.org/lkml/20190731153857.4045-1-pasha.tatashin@xxxxxxxxxx
No linear copy, bug with EL2 reboots.

Pavel Tatashin (8):
  kexec: quiet down kexec reboot
  arm64, mm: transitional tables
  arm64: hibernate: switch to transtional page tables.
  kexec: add machine_kexec_post_load()
  arm64, kexec: move relocation function setup and clean up
  arm64, kexec: add expandable argument to relocation function
  arm64, kexec: configure transitional page table for kexec
  arm64, kexec: enable MMU during kexec relocation

 arch/arm64/Kconfig                     |   4 +
 arch/arm64/include/asm/kexec.h         |  51 ++++-
 arch/arm64/include/asm/pgtable-hwdef.h |   1 +
 arch/arm64/include/asm/trans_table.h   |  68 ++++++
 arch/arm64/kernel/asm-offsets.c        |  14 ++
 arch/arm64/kernel/cpu-reset.S          |   4 +-
 arch/arm64/kernel/cpu-reset.h          |   8 +-
 arch/arm64/kernel/hibernate.c          | 261 ++++++-----------------
 arch/arm64/kernel/machine_kexec.c      | 199 ++++++++++++++----
 arch/arm64/kernel/relocate_kernel.S    | 196 +++++++++---------
 arch/arm64/mm/Makefile                 |   1 +
 arch/arm64/mm/trans_table.c            | 273 +++++++++++++++++++++++++
 kernel/kexec.c                         |   4 +
 kernel/kexec_core.c                    |   8 +-
 kernel/kexec_file.c                    |   4 +
 kernel/kexec_internal.h                |   2 +
 16 files changed, 758 insertions(+), 340 deletions(-)
 create mode 100644 arch/arm64/include/asm/trans_table.h
 create mode 100644 arch/arm64/mm/trans_table.c

-- 
2.22.0