1 files changed, 27 insertions, 13 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d6405e0842b5..45d79ea890ae 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -273,6 +273,7 @@ struct smp_alt_module {
 };
 static LIST_HEAD(smp_alt_modules);
 static DEFINE_SPINLOCK(smp_alt);
+static int smp_mode = 1;        /* protected by smp_alt */
 
 void alternatives_smp_module_add(struct module *mod, char *name,
                                  void *locks, void *locks_end,
@@ -341,12 +342,13 @@ void alternatives_smp_switch(int smp)
 
 #ifdef CONFIG_LOCKDEP
         /*
-         * A not yet fixed binutils section handling bug prevents
-         * alternatives-replacement from working reliably, so turn
-         * it off:
+         * Older binutils section handling bug prevented
+         * alternatives-replacement from working reliably.
+         *
+         * If this still occurs then you should see a hang
+         * or crash shortly after this line:
          */
-        printk("lockdep: not fixing up alternatives.\n");
-        return;
+        printk("lockdep: fixing up alternatives.\n");
 #endif
 
         if (noreplace_smp || smp_alt_once)
@@ -354,21 +356,29 @@ void alternatives_smp_switch(int smp)
         BUG_ON(!smp && (num_online_cpus() > 1));
 
         spin_lock_irqsave(&smp_alt, flags);
-        if (smp) {
+
+        /*
+         * Avoid unnecessary switches because it forces JIT based VMs to
+         * throw away all cached translations, which can be quite costly.
+         */
+        if (smp == smp_mode) {
+                /* nothing */
+        } else if (smp) {
                 printk(KERN_INFO "SMP alternatives: switching to SMP code\n");
-                clear_bit(X86_FEATURE_UP, boot_cpu_data.x86_capability);
-                clear_bit(X86_FEATURE_UP, cpu_data(0).x86_capability);
+                clear_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
+                clear_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
                 list_for_each_entry(mod, &smp_alt_modules, next)
                         alternatives_smp_lock(mod->locks, mod->locks_end,
                                               mod->text, mod->text_end);
         } else {
                 printk(KERN_INFO "SMP alternatives: switching to UP code\n");
-                set_bit(X86_FEATURE_UP, boot_cpu_data.x86_capability);
-                set_bit(X86_FEATURE_UP, cpu_data(0).x86_capability);
+                set_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
+                set_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
                 list_for_each_entry(mod, &smp_alt_modules, next)
                         alternatives_smp_unlock(mod->locks, mod->locks_end,
                                                 mod->text, mod->text_end);
         }
+        smp_mode = smp;
         spin_unlock_irqrestore(&smp_alt, flags);
 }
 
@@ -431,8 +441,9 @@ void __init alternative_instructions(void)
         if (smp_alt_once) {
                 if (1 == num_possible_cpus()) {
                         printk(KERN_INFO "SMP alternatives: switching to UP code\n");
-                        set_bit(X86_FEATURE_UP, boot_cpu_data.x86_capability);
-                        set_bit(X86_FEATURE_UP, cpu_data(0).x86_capability);
+                        set_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
+                        set_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
+
                         alternatives_smp_unlock(__smp_locks, __smp_locks_end,
                                                 _text, _etext);
                 }
@@ -440,7 +451,10 @@ void __init alternative_instructions(void)
                 alternatives_smp_module_add(NULL, "core kernel",
                                             __smp_locks, __smp_locks_end,
                                             _text, _etext);
-                alternatives_smp_switch(0);
+
+                /* Only switch to UP mode if we don't immediately boot others */
+                if (num_possible_cpus() == 1 || setup_max_cpus <= 1)
+                        alternatives_smp_switch(0);
         }
 #endif
         apply_paravirt(__parainstructions, __parainstructions_end);