x86: Introduce x86_cpuinit.early_percpu_clock_init hook

When kvm guest uses kvmclock, it may hang on vcpu hot-plug.
This is caused by an overflow in pvclock_get_nsec_offset,

    u64 delta = tsc - shadow->tsc_timestamp;

which in turn is caused by an undefined values from percpu
hv_clock that hasn't been initialized yet.
Uninitialized clock on being booted cpu is accessed from
   start_secondary
    -> smp_callin
      ->  smp_store_cpu_info
        -> identify_secondary_cpu
          -> mtrr_ap_init
            -> mtrr_restore
              -> stop_machine_from_inactive_cpu
                -> queue_stop_cpus_work
                  ...
                    -> sched_clock
                      -> kvm_clock_read
which is well before x86_cpuinit.setup_percpu_clockev call in
start_secondary, where percpu clock is initialized.

This patch introduces a hook that allows to setup/initialize
per_cpu clock early and avoid overflow due to reading
  - undefined values
  - old values if cpu was offlined and then onlined again

Another possible early user of this clock source is ftrace that
accesses it to get timestamps for ring buffer entries. So if
mtrr_ap_init is moved from identify_secondary_cpu to past
x86_cpuinit.setup_percpu_clockev in start_secondary, ftrace
may cause the same overflow/hang on cpu hot-plug anyway.

More complete description of the problem:
  https://lkml.org/lkml/2012/2/2/101

Credits to Marcelo Tosatti <mtosatti@redhat.com> for hook idea.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/x86_init.h

@@ -145,9 +145,11 @@ struct x86_init_ops {
 /**
  * struct x86_cpuinit_ops - platform specific cpu hotplug setups
  * @setup_percpu_clockev:	set up the per cpu clock event device
+ * @early_percpu_clock_init:	early init of the per cpu clock event device
  */
 struct x86_cpuinit_ops {
 	void (*setup_percpu_clockev)(void);
+	void (*early_percpu_clock_init)(void);
 	void (*fixup_cpu_id)(struct cpuinfo_x86 *c, int node);
 };
 

arch/x86/kernel/kvmclock.c

@@ -144,8 +144,6 @@ static void __cpuinit kvm_setup_secondary_clock(void)
 	 * we shouldn't fail.
 	 */
 	WARN_ON(kvm_register_clock("secondary cpu clock"));
-	/* ok, done with our trickery, call native */
-	setup_secondary_APIC_clock();
 }
 #endif
 
@@ -194,7 +192,7 @@ void __init kvmclock_init(void)
 	x86_platform.get_wallclock = kvm_get_wallclock;
 	x86_platform.set_wallclock = kvm_set_wallclock;
 #ifdef CONFIG_X86_LOCAL_APIC
-	x86_cpuinit.setup_percpu_clockev =
+	x86_cpuinit.early_percpu_clock_init =
 		kvm_setup_secondary_clock;
 #endif
 	machine_ops.shutdown  = kvm_shutdown;

arch/x86/kernel/smpboot.c

@@ -255,6 +255,7 @@ notrace static void __cpuinit start_secondary(void *unused)
 	 * most necessary things.
 	 */
 	cpu_init();
+	x86_cpuinit.early_percpu_clock_init();
 	preempt_disable();
 	smp_callin();
 

arch/x86/kernel/x86_init.c

@@ -91,6 +91,7 @@ struct x86_init_ops x86_init __initdata = {
 };
 
 struct x86_cpuinit_ops x86_cpuinit __cpuinitdata = {
+	.early_percpu_clock_init	= x86_init_noop,
 	.setup_percpu_clockev		= setup_secondary_APIC_clock,
 	.fixup_cpu_id			= x86_default_fixup_cpu_id,
 };