x86, topology: Don't offline CPU0 if any PIC irq can not be migrated out of it

If CONFIG_BOOTPARAM_HOTPLUG_CPU is turned on, CPU0 hotplug feature is enabled
by default.

If CONFIG_BOOTPARAM_HOTPLUG_CPU is not turned on, CPU0 hotplug feature is not
enabled by default. The kernel parameter cpu0_hotplug can enable CPU0 hotplug
feature at boot.

Currently the feature is supported on Intel platforms only.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1352835171-3958-4-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 43 insertions, 7 deletions

diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index 76ee97709a00..0e7b4a7a7fb8 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -30,23 +30,59 @@
 #include <linux/mmzone.h>
 #include <linux/init.h>
 #include <linux/smp.h>
+#include <linux/irq.h>
 #include <asm/cpu.h>
 
 static DEFINE_PER_CPU(struct x86_cpu, cpu_devices);
 
 #ifdef CONFIG_HOTPLUG_CPU
+
+#ifdef CONFIG_BOOTPARAM_HOTPLUG_CPU0
+static int cpu0_hotpluggable = 1;
+#else
+static int cpu0_hotpluggable;
+static int __init enable_cpu0_hotplug(char *str)
+{
+        cpu0_hotpluggable = 1;
+        return 1;
+}
+
+__setup("cpu0_hotplug", enable_cpu0_hotplug);
+#endif
+
 int __ref arch_register_cpu(int num)
 {
+        struct cpuinfo_x86 *c = &cpu_data(num);
+
+        /*
+         * Currently CPU0 is only hotpluggable on Intel platforms. Other
+         * vendors can add hotplug support later.
+         */
+        if (c->x86_vendor != X86_VENDOR_INTEL)
+                cpu0_hotpluggable = 0;
+
         /*
-         * CPU0 cannot be offlined due to several
-         * restrictions and assumptions in kernel. This basically
-         * doesn't add a control file, one cannot attempt to offline
-         * BSP.
+         * Two known BSP/CPU0 dependencies: Resume from suspend/hibernate
+         * depends on BSP. PIC interrupts depend on BSP.
          *
-         * Also certain PCI quirks require not to enable hotplug control
-         * for all CPU's.
+         * If the BSP depencies are under control, one can tell kernel to
+         * enable BSP hotplug. This basically adds a control file and
+         * one can attempt to offline BSP.
          */
-        if (num)
+        if (num == 0 && cpu0_hotpluggable) {
+                unsigned int irq;
+                /*
+                 * We won't take down the boot processor on i386 if some
+                 * interrupts only are able to be serviced by the BSP in PIC.
+                 */
+                for_each_active_irq(irq) {
+                        if (!IO_APIC_IRQ(irq) && irq_has_action(irq)) {
+                                cpu0_hotpluggable = 0;
+                                break;
+                        }
+                }
+        }
+        if (num || cpu0_hotpluggable)
                 per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
 
         return register_cpu(&per_cpu(cpu_devices, num).cpu, num);