[V5 PATCH 4/4] x86/apic: Introduce apic_extnmi boot option

[Hidehiro Kawai]

This patch introduces new boot option, apic_extnmi:

 apic_extnmi={ bsp | all | none}

The default value is "bsp" and this is the current behavior; only
BSP receives external NMI.  "all" allows external NMIs to be
broadcast to all CPUs.  This would raise the success rate of panic
on NMI when BSP hangs up in NMI context or the external NMI is
swallowed by other NMI handlers on BSP.  If you specified "none",
any CPUs don't receive external NMIs.  This is useful for dump
capture kernel so that it wouldn't be shot down while saving a
crash dump.

V5:
- Rename the option from "noextnmi" to "apic_extnmi"
- Add apic_extnmi=all feature
- Fix the wrong documentation about "noextnmi" (apic_extnmi=none)

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez at hitachi.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: Jonathan Corbet <corbet at lwn.net>
---
 Documentation/kernel-parameters.txt |    9 +++++++++
 arch/x86/include/asm/apic.h         |    5 +++++
 arch/x86/kernel/apic/apic.c         |   31 ++++++++++++++++++++++++++++++-
 3 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index f8aae63..ceed3bc 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -472,6 +472,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Change the amount of debugging information output
 			when initialising the APIC and IO-APIC components.
 
+	apic_extnmi=	[APIC,X86] External NMI delivery setting
+			Format: { bsp (default) | all | none }
+			bsp:  External NMI is delivered to only CPU 0
+			all:  External NMIs are broadcast to all CPUs as a
+			      backup of CPU 0
+			none: External NMI is masked for all CPUs. This is
+			      useful so that a dump capture kernel won't be
+			      shot down by NMI
+
 	autoconf=	[IPV6]
 			See Documentation/networking/ipv6.txt.
 
diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 7f62ad4..c80f6b6 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -23,6 +23,11 @@
 #define APIC_VERBOSE 1
 #define APIC_DEBUG   2
 
+/* Macros for apic_extnmi which controls external NMI masking */
+#define APIC_EXTNMI_BSP		0 /* Default */
+#define APIC_EXTNMI_ALL		1
+#define APIC_EXTNMI_NONE	2
+
 /*
  * Define the default level of output to be very little
  * This can be turned up by using apic=verbose for more
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 2f69e3b..a2a8074 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -82,6 +82,12 @@ physid_mask_t phys_cpu_present_map;
 static unsigned int disabled_cpu_apicid __read_mostly = BAD_APICID;
 
 /*
+ * This variable controls which CPUs receive external NMIs.  By default,
+ * external NMIs are delivered to only BSP.
+ */
+static int apic_extnmi = APIC_EXTNMI_BSP;
+
+/*
  * Map cpu index to physical APIC ID
  */
 DEFINE_EARLY_PER_CPU_READ_MOSTLY(u16, x86_cpu_to_apicid, BAD_APICID);
@@ -1161,6 +1167,8 @@ void __init init_bsp_APIC(void)
 	value = APIC_DM_NMI;
 	if (!lapic_is_integrated())		/* 82489DX */
 		value |= APIC_LVT_LEVEL_TRIGGER;
+	if (apic_extnmi == APIC_EXTNMI_NONE)
+		value |= APIC_LVT_MASKED;
 	apic_write(APIC_LVT1, value);
 }
 
@@ -1380,7 +1388,8 @@ void setup_local_APIC(void)
 	/*
 	 * only the BP should see the LINT1 NMI signal, obviously.
 	 */
-	if (!cpu)
+	if ((!cpu && apic_extnmi != APIC_EXTNMI_NONE) ||
+	    apic_extnmi == APIC_EXTNMI_ALL)
 		value = APIC_DM_NMI;
 	else
 		value = APIC_DM_NMI | APIC_LVT_MASKED;
@@ -2548,3 +2557,23 @@ static int __init apic_set_disabled_cpu_apicid(char *arg)
 	return 0;
 }
 early_param("disable_cpu_apicid", apic_set_disabled_cpu_apicid);
+
+static int __init apic_set_extnmi(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	if (strcmp("all", arg) == 0)
+		apic_extnmi = APIC_EXTNMI_ALL;
+	else if (strcmp("none", arg) == 0)
+		apic_extnmi = APIC_EXTNMI_NONE;
+	else if (strcmp("bsp", arg) == 0)
+		apic_extnmi = APIC_EXTNMI_BSP;
+	else {
+		pr_warn("Unknown external NMI delivery mode `%s' is ignored\n",
+			arg);
+	}
+
+	return 0;
+}
+early_param("apic_extnmi", apic_set_extnmi);