1 files changed, 14 insertions, 2 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index cdc43242da92..318a4f9b7ece 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,
@@ -354,7 +355,14 @@ 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_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
                 clear_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
@@ -369,6 +377,7 @@ void alternatives_smp_switch(int smp)
                         alternatives_smp_unlock(mod->locks, mod->locks_end,
                                                 mod->text, mod->text_end);
         }
+        smp_mode = smp;
         spin_unlock_irqrestore(&smp_alt, flags);
 }
 
@@ -441,7 +450,10 @@ 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);