[PATCH v3 38/41] x86/paravirt: kvmclock: Setup kvmclock early iff it's sched_clock

[Sean Christopherson <seanjc@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 arch/x86/include/asm/timer.h    |  8 ++++++--
 arch/x86/include/asm/x86_init.h |  2 --
 arch/x86/kernel/kvmclock.c      | 13 ++++++-------
 arch/x86/kernel/smpboot.c       |  3 ++-
 arch/x86/kernel/tsc.c           | 16 +++++++++++++++-
 arch/x86/kernel/x86_init.c      |  1 -
 6 files changed, 29 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
index ca5c95d48c03..ab1271bd9c3b 100644
--- a/arch/x86/include/asm/timer.h
+++ b/arch/x86/include/asm/timer.h
@@ -15,14 +15,18 @@ extern bool using_native_sched_clock(void);
 
 #ifdef CONFIG_PARAVIRT
 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);
 }
+void paravirt_sched_clock_start_secondary(void);
+#else
+static inline void paravirt_sched_clock_start_secondary(void) { }
 #endif
 
 /*
diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
index 6c8a6ead84f6..d1b3f18ea41f 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 0578bc448b1b..62c8ea2e6769 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -127,12 +127,13 @@ 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)
 {
@@ -352,7 +353,8 @@ static __init void 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();
@@ -437,9 +439,6 @@ 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/smpboot.c b/arch/x86/kernel/smpboot.c
index 294a8ea60298..318ae70e5e7b 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -78,6 +78,7 @@
 #include <asm/io_apic.h>
 #include <asm/fpu/api.h>
 #include <asm/setup.h>
+#include <asm/timer.h>
 #include <asm/uv/uv.h>
 #include <asm/microcode.h>
 #include <asm/i8259.h>
@@ -275,7 +276,7 @@ static void notrace __noendbr 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/tsc.c b/arch/x86/kernel/tsc.c
index 7a261214fa3e..f78e86494dec 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -280,8 +280,19 @@ bool using_native_sched_clock(void)
        return static_call_query(pv_sched_clock) == native_sched_clock;
 }
 
+#ifdef CONFIG_SMP
+static void (*pv_sched_clock_start_secondary)(void) __ro_after_init;
+
+void paravirt_sched_clock_start_secondary(void)
+{
+       if (pv_sched_clock_start_secondary)
+               pv_sched_clock_start_secondary();
+}
+#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
@@ -298,6 +309,9 @@ 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;
 }
 #else
diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
index ebefb77c37bb..cbb5ee613ed5 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,
 };
-- 
2.54.0.563.g4f69b47b94-goog