linux-kernel - [PATCH v19 7/7] x86/crash: optimize cpu changes

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Date:   Mon,  6 Mar 2023 11:22:28 -0500
From:   Eric DeVolder <eric.devolder@...cle.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com, eric.devolder@...cle.com
Subject: [PATCH v19 7/7] x86/crash: optimize cpu changes

This patch is dependent upon the patch 'crash: change
crash_prepare_elf64_headers() to for_each_possible_cpu()'. With that
patch, crash_prepare_elf64_headers() writes out an ELF cpu PT_NOTE
for all possible cpus, thus further cpu changes to the elfcorehdr
are not needed.

This change works for kexec_file_load() and kexec_load() syscalls.
For kexec_file_load(), crash_prepare_elf64_headers() is utilized
directly and thus all ELF cpu PT_NOTEs are in the elfcorehdr already.
This is the kimage->file_mode term.
For kexec_load() syscall, one cpu or memory change will cause the
elfcorehdr to be updated via crash_prepare_elf64_headers() and at
that point all ELF cpu PT_NOTEs are in the elfcorehdr. This is the
kimage->elfcorehdr_updated term.

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.

Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
---
 arch/x86/kernel/crash.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index c9aaec9de775..82ea2b1bdc61 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -471,6 +471,16 @@ 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. This works for both kexec_load()
+	 * and kexec_file_load() syscalls.
+	 */
+	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.
-- 
2.31.1