[RFC v1 2/8] crash hp: Introduce CRASH_HOTPLUG configuration options

[Eric DeVolder]

The bulk of the support for CPU and memory hotplug support for
crash is controlled by the CRASH_HOTPLUG configuration option,
introduced by this patch.

The CRASH_HOTPLUG_ELFCOREHDR_SZ related configuration option is
also introduced with this patch.

Signed-off-by: Eric DeVolder <eric.devolder at oracle.com>
---
 arch/x86/Kconfig | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 95dd1ee01546..5feb829adb60 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2046,6 +2046,32 @@ config CRASH_DUMP
 	  (CONFIG_RELOCATABLE=y).
 	  For more details see Documentation/admin-guide/kdump/kdump.rst
 
+config CRASH_HOTPLUG
+	bool "kernel updates of crash elfcorehdr"
+	depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG) && KEXEC_FILE
+	help
+	  Enable the kernel to update the crash elfcorehdr (which contains
+	  the list of CPUs and memory regions) directly when hot plug/unplug
+	  of CPUs or memory. Otherwise userspace must monitor these hot
+	  plug/unplug change notifications via udev in order to
+	  unload-then-reload the crash kernel so that the list of CPUs and
+	  memory regions is kept up-to-date. Note that the udev CPU and
+	  memory change notifications still occur (however, userspace is not
+	  required to monitor for crash dump purposes).
+
+config CRASH_HOTPLUG_ELFCOREHDR_SZ
+	depends on CRASH_HOTPLUG
+	int
+	default 131072
+	help
+	  Specify the maximum size of the elfcorehdr buffer/segment.
+	  The 128KiB default is sized so that it can accommodate 2048
+	  Elf64_Phdr, where each Phdr represents either a CPU or a
+	  region of memory.
+	  For example, this size can accommodate hotplugging a machine
+	  with up to 1024 CPUs and up to 1024 memory regions (e.g. 1TiB
+	  with 1024 1GiB memory DIMMs).
+
 config KEXEC_JUMP
 	bool "kexec jump"
 	depends on KEXEC && HIBERNATION
-- 
2.27.0