3 files changed, 24 insertions, 10 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);
diff --git a/include/linux/smp.h b/include/linux/smp.h
index c25e66bcecf3..55232ccf9cfd 100644
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -78,6 +78,8 @@ int on_each_cpu(void (*func) (void *info), void *info, int retry, int wait);
  */
 void smp_prepare_boot_cpu(void);
 
+extern unsigned int setup_max_cpus;
+
 #else /* !SMP */
 
 /*
diff --git a/init/main.c b/init/main.c
index 3f8aba291ed3..5843fe996703 100644
--- a/init/main.c
+++ b/init/main.c
@@ -128,7 +128,7 @@ static char *ramdisk_execute_command;
 
 #ifdef CONFIG_SMP
 /* Setup configured maximum number of CPUs to activate */
-static unsigned int __initdata max_cpus = NR_CPUS;
+unsigned int __initdata setup_max_cpus = NR_CPUS;
 
 /*
  * Setup routine for controlling SMP activation
@@ -146,7 +146,7 @@ static inline void disable_ioapic_setup(void) {};
 
 static int __init nosmp(char *str)
 {
-        max_cpus = 0;
+        setup_max_cpus = 0;
         disable_ioapic_setup();
         return 0;
 }
@@ -155,8 +155,8 @@ early_param("nosmp", nosmp);
 
 static int __init maxcpus(char *str)
 {
-        get_option(&str, &max_cpus);
-        if (max_cpus == 0)
+        get_option(&str, &setup_max_cpus);
+        if (setup_max_cpus == 0)
                 disable_ioapic_setup();
 
         return 0;
@@ -164,7 +164,7 @@ static int __init maxcpus(char *str)
 
 early_param("maxcpus", maxcpus);
 #else
-#define max_cpus NR_CPUS
+#define setup_max_cpus NR_CPUS
 #endif
 
 /*
@@ -393,7 +393,7 @@ static void __init smp_init(void)
 
         /* FIXME: This should be done in userspace --RR */
         for_each_present_cpu(cpu) {
-                if (num_online_cpus() >= max_cpus)
+                if (num_online_cpus() >= setup_max_cpus)
                         break;
                 if (!cpu_online(cpu))
                         cpu_up(cpu);
@@ -401,7 +401,7 @@ static void __init smp_init(void)
 
         /* Any cleanup work */
         printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus());
-        smp_cpus_done(max_cpus);
+        smp_cpus_done(setup_max_cpus);
 }
 
 #endif
@@ -824,7 +824,7 @@ static int __init kernel_init(void * unused)
         __set_special_pids(1, 1);
         cad_pid = task_pid(current);
 
-        smp_prepare_cpus(max_cpus);
+        smp_prepare_cpus(setup_max_cpus);
 
         do_pre_smp_initcalls();