[PATCH v7 0/8] In kernel handling of CPU hotplug events for crash kernel

[Sourabh Jain]

This patch series implements the crash hotplug handler on PowerPC introduced
by https://lkml.org/lkml/2022/12/9/520 patch series.

The Problem:
============
Post hotplug/DLPAR events the capture kernel holds stale information about the
system. Dump collection with stale capture kernel might end up in dump capture
failure or an inaccurate dump collection.

Existing solution:
==================
The existing solution to keep the capture kernel up-to-date by monitoring
hotplug event via udev rule and trigger a full capture kernel reload for
every hotplug event.

Shortcomings:
------------------------------------------------
- Leaves a window where kernel crash might not lead to a successful dump
  collection.
- Reloading all kexec components for each hotplug is inefficient.
- udev rules are prone to races if hotplug events are frequent.

More about issues with an existing solution is posted here:
 - https://lkml.org/lkml/2020/12/14/532
 - https://lists.ozlabs.org/pipermail/linuxppc-dev/2022-February/240254.html

Proposed Solution:
==================
Instead of reloading all kexec segments on hotplug event, this patch series
focuses on updating only the relevant kexec segment. Once the kexec segments
are loaded in the kernel reserved area then an arch-specific hotplug handler
will update the relevant kexec segment based on hotplug event type.

---
Changelog:

v6 -> v7
  - added a new config to configure this feature
  - pass hotplug action type to arch specific handler

v5 -> v6
  - Added crash memory hotplug support

v4 -> v5:
  - Replace COFNIG_CRASH_HOTPLUG with CONFIG_HOTPLUG_CPU.
  - Move fdt segment identification for kexec_load case to load path
    instead of crash hotplug handler
  - Keep new attribute defined under kimage_arch to track FDT segment
    under CONFIG_HOTPLUG_CPU config.

v3 -> v4:
  - Update the logic to find the additional space needed for hotadd CPUs post
    kexec load. Refer "[RFC v4 PATCH 4/5] powerpc/crash hp: add crash hotplug
    support for kexec_file_load" patch to know more about the change.
  - Fix a couple of typo.
  - Replace pr_err to pr_info_once to warn user about memory hotplug
    support.
  - In crash hotplug handle exit the for loop if FDT segment is found.

v2 -> v3
  - Move fdt_index and fdt_index_vaild variables to kimage_arch struct.
  - Rebase patche on top of https://lkml.org/lkml/2022/3/3/674 [v5]
  - Fixed warning reported by checpatch script

v1 -> v2:
  - Use generic hotplug handler introduced by https://lkml.org/lkml/2022/2/9/1406, a
    significant change from v1.
---

Sourabh Jain (8):
  powerpc/kexec: turn some static helper functions public
  powerpc/crash hp: introduce a new config option CRASH_HOTPLUG
  powerpc/crash: update kimage_arch struct
  crash: add phdr for possible CPUs in elfcorehdr
  crash: pass hotplug action type to arch crash hotplug handler
  powerpc/crash: add crash CPU hotplug support
  crash: forward memory_notify args to arch crash hotplug handler
  powerpc/kexec: add crash memory hotplug support

 arch/powerpc/Kconfig                    |  12 +
 arch/powerpc/include/asm/kexec.h        |  17 ++
 arch/powerpc/include/asm/kexec_ranges.h |   1 +
 arch/powerpc/kexec/core_64.c            | 290 ++++++++++++++++++++++++
 arch/powerpc/kexec/elf_64.c             |  73 +++++-
 arch/powerpc/kexec/file_load_64.c       | 179 ++-------------
 arch/powerpc/kexec/ranges.c             |  60 +++++
 arch/x86/include/asm/kexec.h            |   3 +-
 arch/x86/kernel/crash.c                 |   5 +-
 include/linux/kexec.h                   |   6 +-
 kernel/crash_core.c                     |  23 +-
 11 files changed, 491 insertions(+), 178 deletions(-)

-- 
2.39.0