linux-kernel - [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry

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Date:   Fri,  5 Aug 2022 10:30:09 -0700
From:   ira.weiny@...el.com
To:     Rik van Riel <riel@...riel.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry

From: Ira Weiny <ira.weiny@...el.com>

The CPU information of an exception is useful in determining where bad
CPUs are in a large data center.

Define arch_{save|restore}_auxiliary_pt_regs() and set
ARCH_HAS_PTREGS_AUXILIARY default to yes.

Store the CPU on exception entry and use it later.

Cc: Rik van Riel <riel@...riel.com>
Suggested-by: Borislav Petkov <bp@...en8.de>
Suggested-by: Dave Hansen <dave.hansen@...el.com>
Signed-off-by: Ira Weiny <ira.weiny@...el.com>
---
 arch/x86/Kconfig                    |  2 +-
 arch/x86/include/asm/entry-common.h | 12 ++++++++++++
 arch/x86/include/asm/ptrace.h       |  1 +
 arch/x86/mm/fault.c                 |  4 ++--
 4 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b35f6a472e09..707650a6ecb2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1876,7 +1876,7 @@ config X86_INTEL_MEMORY_PROTECTION_KEYS
 
 config ARCH_HAS_PTREGS_AUXILIARY
 	depends on X86_64
-	bool
+	def_bool y
 
 choice
 	prompt "TSX enable mode"
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 674ed46d3ced..eb145106929a 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -95,4 +95,16 @@ static __always_inline void arch_exit_to_user_mode(void)
 }
 #define arch_exit_to_user_mode arch_exit_to_user_mode
 
+#ifdef CONFIG_ARCH_HAS_PTREGS_AUXILIARY
+
+static inline void arch_save_aux_pt_regs(struct pt_regs *regs)
+{
+	struct pt_regs_auxiliary *aux_pt_regs = &to_extended_pt_regs(regs)->aux;
+
+	aux_pt_regs->cpu = raw_smp_processor_id();
+}
+#define arch_save_aux_pt_regs arch_save_aux_pt_regs
+
+#endif
+
 #endif
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 5a9c85893459..b403b469996f 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -97,6 +97,7 @@ struct pt_regs {
  * ARCH_HAS_PTREGS_AUXILIARY.  Failure to do so will result in a build failure.
  */
 struct pt_regs_auxiliary {
+	int cpu;
 };
 
 struct pt_regs_extended {
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 82cf23975aa1..5df99fe49494 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -768,9 +768,9 @@ static inline void
 show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 		unsigned long address, struct task_struct *tsk)
 {
+	struct pt_regs_auxiliary *aux_pt_regs = &to_extended_pt_regs(regs)->aux;
 	const char *loglvl = task_pid_nr(tsk) > 1 ? KERN_INFO : KERN_EMERG;
-	/* This is a racy snapshot, but it's better than nothing. */
-	int cpu = raw_smp_processor_id();
+	int cpu = aux_pt_regs->cpu;
 
 	if (!unhandled_signal(tsk, SIGSEGV))
 		return;
-- 
2.35.3