x86/crash: optimize CPU changes

crash_prepare_elf64_headers() writes into the elfcorehdr an ELF PT_NOTE
for all possible CPUs.  As such, subsequent changes to CPUs (ie.  hot
un/plug, online/offline) do not need to rewrite the elfcorehdr.

The kimage->file_mode term covers kdump images loaded via the
kexec_file_load() syscall.  Since crash_prepare_elf64_headers() wrote the
initial elfcorehdr, no update to the elfcorehdr is needed for CPU changes.

The kimage->elfcorehdr_updated term covers kdump images loaded via the
kexec_load() syscall.  At least one memory or CPU change must occur to
cause crash_prepare_elf64_headers() to rewrite the elfcorehdr. 
Afterwards, no update to the elfcorehdr is needed for CPU changes.

This code is intentionally *NOT* hoisted into crash_handle_hotplug_event()
as it would prevent the arch-specific handler from running for CPU
changes.  This would break PPC, for example, which needs to update other
information besides the elfcorehdr, on CPU changes.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Eric DeVolder <[email protected]>
Reviewed-by: Sourabh Jain <[email protected]>
Acked-by: Hari Bathini <[email protected]>
Acked-by: Baoquan He <[email protected]>
Cc: Akhil Raj <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Borislav Petkov (AMD) <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Dave Young <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Mimi Zohar <[email protected]>
Cc: Naveen N. Rao <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Takashi Iwai <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Thomas Weißschuh <[email protected]>
Cc: Valentin Schneider <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Eric DeVolder

arch/x86/kernel/crash.c

@@ -469,6 +469,16 @@ void arch_crash_handle_hotplug_event(struct kimage *image)
 	unsigned long mem, memsz;
 	unsigned long elfsz = 0;
 
+	/*
+	 * As crash_prepare_elf64_headers() has already described all
+	 * possible CPUs, there is no need to update the elfcorehdr
+	 * for additional CPU changes.
+	 */
+	if ((image->file_mode || image->elfcorehdr_updated) &&
+		((image->hp_action == KEXEC_CRASH_HP_ADD_CPU) ||
+		(image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU)))
+		return;
+
 	/*
 	 * Create the new elfcorehdr reflecting the changes to CPU and/or
 	 * memory resources.