1 files changed, 24 insertions, 9 deletions

diff --git a/arch/x86_64/kernel/genapic.c b/arch/x86_64/kernel/genapic.c
index 69b9c25a8fc4..30c843a5efdd 100644
--- a/arch/x86_64/kernel/genapic.c
+++ b/arch/x86_64/kernel/genapic.c
@@ -31,6 +31,7 @@ u8 x86_cpu_to_log_apicid[NR_CPUS] = { [0 ... NR_CPUS-1] = BAD_APICID };
 
 extern struct genapic apic_cluster;
 extern struct genapic apic_flat;
+extern struct genapic apic_physflat;
 
 struct genapic *genapic = &apic_flat;
 
@@ -44,12 +45,7 @@ void __init clustered_apic_check(void)
         u8 clusters, max_cluster;
         u8 id;
         u8 cluster_cnt[NUM_APIC_CLUSTERS];
-
-        if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
-                /* AMD always uses flat mode right now */
-                genapic = &apic_flat;
-                goto print;
-        }
+        int num_cpus = 0;
 
 #if defined(CONFIG_ACPI_BUS)
         /*
@@ -64,15 +60,34 @@ void __init clustered_apic_check(void)
 #endif
 
         memset(cluster_cnt, 0, sizeof(cluster_cnt));
-
         for (i = 0; i < NR_CPUS; i++) {
                 id = bios_cpu_apicid[i];
-                if (id != BAD_APICID)
-                        cluster_cnt[APIC_CLUSTERID(id)]++;
+                if (id == BAD_APICID)
+                        continue;
+                num_cpus++;
+                cluster_cnt[APIC_CLUSTERID(id)]++;
         }
 
+        /* Don't use clustered mode on AMD platforms. */
+        if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
+                genapic = &apic_physflat;
+#ifndef CONFIG_CPU_HOTPLUG
+                /* In the CPU hotplug case we cannot use broadcast mode
+                   because that opens a race when a CPU is removed.
+                   Stay at physflat mode in this case.
+                   It is bad to do this unconditionally though. Once
+                   we have ACPI platform support for CPU hotplug
+                   we should detect hotplug capablity from ACPI tables and
+                   only do this when really needed. -AK */
+                if (num_cpus <= 8)
+                        genapic = &apic_flat;
+#endif
+                goto print;
+        }
+
         clusters = 0;
         max_cluster = 0;
+
         for (i = 0; i < NUM_APIC_CLUSTERS; i++) {
                 if (cluster_cnt[i] > 0) {
                         ++clusters;