x86 mmiotrace: dynamically disable non-boot CPUs

From 8979ee55cb6a429c4edd72ebec2244b849f6a79a Mon Sep 17 00:00:00 2001
From: Pekka Paalanen <pq@iki.fi>
Date: Sat, 12 Apr 2008 00:18:57 +0300

Mmiotrace is not reliable with multiple CPUs and may
miss events. Drop to single CPU when mmiotrace is activated.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 61 insertions, 0 deletions

diff --git a/arch/x86/mm/mmio-mod.c b/arch/x86/mm/mmio-mod.c
index 6d6cac84c045..1f77d8532037 100644
--- a/arch/x86/mm/mmio-mod.c
+++ b/arch/x86/mm/mmio-mod.c
@@ -32,6 +32,7 @@
 #include <asm/e820.h> /* for ISA_START_ADDRESS */
 #include <asm/atomic.h>
 #include <linux/percpu.h>
+#include <linux/cpu.h>
 
 #include "pf_in.h"
 
@@ -400,6 +401,64 @@ static void clear_trace_list(void)
         }
 }
 
+#ifdef CONFIG_HOTPLUG_CPU
+static cpumask_t downed_cpus;
+
+static void enter_uniprocessor(void)
+{
+        int cpu;
+        int err;
+
+        get_online_cpus();
+        downed_cpus = cpu_online_map;
+        cpu_clear(first_cpu(cpu_online_map), downed_cpus);
+        if (num_online_cpus() > 1)
+                pr_notice(NAME "Disabling non-boot CPUs...\n");
+        put_online_cpus();
+
+        for_each_cpu_mask(cpu, downed_cpus) {
+                err = cpu_down(cpu);
+                if (!err) {
+                        pr_info(NAME "CPU%d is down.\n", cpu);
+                } else {
+                        pr_err(NAME "Error taking CPU%d down: %d\n", cpu, err);
+                }
+        }
+        if (num_online_cpus() > 1)
+                pr_warning(NAME "multiple CPUs still online, "
+                                                "may miss events.\n");
+}
+
+static void leave_uniprocessor(void)
+{
+        int cpu;
+        int err;
+
+        if (cpus_weight(downed_cpus) == 0)
+                return;
+        pr_notice(NAME "Re-enabling CPUs...\n");
+        for_each_cpu_mask(cpu, downed_cpus) {
+                err = cpu_up(cpu);
+                if (!err)
+                        pr_info(NAME "enabled CPU%d.\n", cpu);
+                else
+                        pr_err(NAME "cannot re-enable CPU%d: %d\n", cpu, err);
+        }
+}
+
+#else /* !CONFIG_HOTPLUG_CPU */
+static void enter_uniprocessor(void)
+{
+        if (num_online_cpus() > 1)
+                pr_warning(NAME "multiple CPUs are online, may miss events. "
+                        "Suggest booting with maxcpus=1 kernel argument.\n");
+}
+
+static void leave_uniprocessor(void)
+{
+}
+#endif
+
 #if 0 /* XXX: out of order */
 static struct file_operations fops_marker = {
         .owner =        THIS_MODULE,
@@ -422,6 +481,7 @@ void enable_mmiotrace(void)
 
         if (nommiotrace)
                 pr_info(NAME "MMIO tracing disabled.\n");
+        enter_uniprocessor();
         spin_lock_irq(&trace_lock);
         atomic_inc(&mmiotrace_enabled);
         spin_unlock_irq(&trace_lock);
@@ -442,6 +502,7 @@ void disable_mmiotrace(void)
         spin_unlock_irq(&trace_lock);
 
         clear_trace_list(); /* guarantees: no more kmmio callbacks */
+        leave_uniprocessor();
         if (marker_file) {
                 debugfs_remove(marker_file);
                 marker_file = NULL;