[PATCH v2 38/38] x86/paravirt: kvmclock: Setup kvmclock early iff it's sched_clock

[Sean Christopherson <seanjc@google.com>]

Rework the seemingly generic x86_cpuinit_ops.early_percpu_clock_init hook
into a dedicated PV sched_clock hook, as the only reason the hook exists
is to allow kvmclock to enable its PV clock on secondary CPUs before the
kernel tries to reference sched_clock, e.g. when grabbing a timestamp for
printk.

Rearranging the hook doesn't exactly reduce complexity; arguably it does
the opposite.  But as-is, it's practically impossible to understand *why*
kvmclock needs to do early configuration.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/paravirt.h | 10 ++++++++--
 arch/x86/include/asm/x86_init.h |  2 --
 arch/x86/kernel/kvmclock.c      | 13 ++++++-------
 arch/x86/kernel/paravirt.c      | 18 +++++++++++++++++-
 arch/x86/kernel/smpboot.c       |  2 +-
 arch/x86/kernel/x86_init.c      |  1 -
 6 files changed, 32 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 5de31b22aa5f..8550262fc710 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -29,13 +29,14 @@ DECLARE_STATIC_CALL(pv_steal_clock, dummy_steal_clock);
 DECLARE_STATIC_CALL(pv_sched_clock, dummy_sched_clock);
 
 int __init __paravirt_set_sched_clock(u64 (*func)(void), bool stable,
-				      void (*save)(void), void (*restore)(void));
+				      void (*save)(void), void (*restore)(void),
+				      void (*start_secondary));
 
 static __always_inline void paravirt_set_sched_clock(u64 (*func)(void),
 						     void (*save)(void),
 						     void (*restore)(void))
 {
-	(void)__paravirt_set_sched_clock(func, true, save, restore);
+	(void)__paravirt_set_sched_clock(func, true, save, restore, NULL);
 }
 
 static __always_inline u64 paravirt_sched_clock(void)
@@ -43,6 +44,8 @@ static __always_inline u64 paravirt_sched_clock(void)
 	return static_call(pv_sched_clock)();
 }
 
+void paravirt_sched_clock_start_secondary(void);
+
 struct static_key;
 extern struct static_key paravirt_steal_enabled;
 extern struct static_key paravirt_steal_rq_enabled;
@@ -756,6 +759,9 @@ void native_pv_lock_init(void) __init;
 static inline void native_pv_lock_init(void)
 {
 }
+static inline void paravirt_sched_clock_start_secondary(void)
+{
+}
 #endif
 #endif /* !CONFIG_PARAVIRT */
 
diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
index 213cf5379a5a..e3456def5aea 100644
--- a/arch/x86/include/asm/x86_init.h
+++ b/arch/x86/include/asm/x86_init.h
@@ -187,13 +187,11 @@ 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
  * @fixup_cpu_id:		fixup function for cpuinfo_x86::topo.pkg_id
  * @parallel_bringup:		Parallel bringup control
  */
 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);
 	bool parallel_bringup;
 };
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 280bb964f30a..11f078b91f22 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -126,12 +126,13 @@ static void kvm_save_sched_clock_state(void)
 	kvmclock_disable();
 }
 
-#ifdef CONFIG_SMP
-static void kvm_setup_secondary_clock(void)
+static void kvm_setup_secondary_sched_clock(void)
 {
+	if (WARN_ON_ONCE(!IS_ENABLED(CONFIG_SMP)))
+		return;
+
 	kvm_register_clock("secondary cpu, sched_clock setup");
 }
-#endif
 
 static void kvm_restore_sched_clock_state(void)
 {
@@ -367,7 +368,8 @@ static void __init kvm_sched_clock_init(bool stable)
 {
 	if (__paravirt_set_sched_clock(kvm_sched_clock_read, stable,
 				       kvm_save_sched_clock_state,
-				       kvm_restore_sched_clock_state))
+				       kvm_restore_sched_clock_state,
+				       kvm_setup_secondary_sched_clock))
 		return;
 
 	kvm_sched_clock_offset = kvm_clock_read();
@@ -452,9 +454,6 @@ void __init kvmclock_init(void)
 
 	x86_platform.get_wallclock = kvm_get_wallclock;
 	x86_platform.set_wallclock = kvm_set_wallclock;
-#ifdef CONFIG_SMP
-	x86_cpuinit.early_percpu_clock_init = kvm_setup_secondary_clock;
-#endif
 	kvm_get_preset_lpj();
 
 	clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index c538c608d9fb..f93278ddb1d2 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -86,8 +86,13 @@ static u64 native_steal_clock(int cpu)
 DEFINE_STATIC_CALL(pv_steal_clock, native_steal_clock);
 DEFINE_STATIC_CALL(pv_sched_clock, native_sched_clock);
 
+#ifdef CONFIG_SMP
+static void (*pv_sched_clock_start_secondary)(void) __ro_after_init;
+#endif
+
 int __init __paravirt_set_sched_clock(u64 (*func)(void), bool stable,
-				      void (*save)(void), void (*restore)(void))
+				      void (*save)(void), void (*restore)(void),
+				      void (*start_secondary))
 {
 	/*
 	 * Don't replace TSC with a PV clock when running as a CoCo guest and
@@ -104,9 +109,20 @@ int __init __paravirt_set_sched_clock(u64 (*func)(void), bool stable,
 	static_call_update(pv_sched_clock, func);
 	x86_platform.save_sched_clock_state = save;
 	x86_platform.restore_sched_clock_state = restore;
+#ifdef CONFIG_SMP
+	pv_sched_clock_start_secondary = start_secondary;
+#endif
 	return 0;
 }
 
+#ifdef CONFIG_SMP
+void paravirt_sched_clock_start_secondary(void)
+{
+	if (pv_sched_clock_start_secondary)
+		pv_sched_clock_start_secondary();
+}
+#endif
+
 /* These are in entry.S */
 static struct resource reserve_ioports = {
 	.start = 0,
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index c10850ae6f09..e6fff67dd264 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -278,7 +278,7 @@ static void notrace start_secondary(void *unused)
 	cpu_init();
 	fpu__init_cpu();
 	rcutree_report_cpu_starting(raw_smp_processor_id());
-	x86_cpuinit.early_percpu_clock_init();
+	paravirt_sched_clock_start_secondary();
 
 	ap_starting();
 
diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
index 0a2bbd674a6d..1d4cf071c74b 100644
--- a/arch/x86/kernel/x86_init.c
+++ b/arch/x86/kernel/x86_init.c
@@ -128,7 +128,6 @@ struct x86_init_ops x86_init __initdata = {
 };
 
 struct x86_cpuinit_ops x86_cpuinit = {
-	.early_percpu_clock_init	= x86_init_noop,
 	.setup_percpu_clockev		= setup_secondary_APIC_clock,
 	.parallel_bringup		= true,
 };
-- 
2.48.1.711.g2feabab25a-goog