[PATCH v2 08/38] clocksource: hyper-v: Register sched_clock save/restore iff it's necessary

[Sean Christopherson <seanjc@google.com>]

Register the Hyper-V timer callbacks or saving/restoring its PV sched_clock
if and only if the timer is actually being used for sched_clock.
Currently, Hyper-V overrides the save/restore hooks if the reference TSC
available, whereas the Hyper-V timer code only overrides sched_clock if
the reference TSC is available *and* it's not invariant.  The flaw is
effectively papered over by invoking the "old" save/restore callbacks as
part of save/restore, but that's unnecessary and fragile.

To avoid introducing more complexity, and to allow for additional cleanups
of the PV sched_clock code, move the save/restore hooks and logic into
hyperv_timer.c and simply wire up the hooks when overriding sched_clock
itself.

Note, while the Hyper-V timer code is intended to be architecture neutral,
CONFIG_PARAVIRT is firmly x86-only, i.e. adding a small amount of x86
specific code (which will be reduced in future cleanups) doesn't
meaningfully pollute generic code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kernel/cpu/mshyperv.c     | 58 ------------------------------
 drivers/clocksource/hyperv_timer.c | 50 ++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 58 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index aa60491bf738..174f6a71c899 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -223,63 +223,6 @@ static void hv_machine_crash_shutdown(struct pt_regs *regs)
 	hyperv_cleanup();
 }
 #endif /* CONFIG_CRASH_DUMP */
-
-static u64 hv_ref_counter_at_suspend;
-static void (*old_save_sched_clock_state)(void);
-static void (*old_restore_sched_clock_state)(void);
-
-/*
- * Hyper-V clock counter resets during hibernation. Save and restore clock
- * offset during suspend/resume, while also considering the time passed
- * before suspend. This is to make sure that sched_clock using hv tsc page
- * based clocksource, proceeds from where it left off during suspend and
- * it shows correct time for the timestamps of kernel messages after resume.
- */
-static void save_hv_clock_tsc_state(void)
-{
-	hv_ref_counter_at_suspend = hv_read_reference_counter();
-}
-
-static void restore_hv_clock_tsc_state(void)
-{
-	/*
-	 * Adjust the offsets used by hv tsc clocksource to
-	 * account for the time spent before hibernation.
-	 * adjusted value = reference counter (time) at suspend
-	 *                - reference counter (time) now.
-	 */
-	hv_adj_sched_clock_offset(hv_ref_counter_at_suspend - hv_read_reference_counter());
-}
-
-/*
- * Functions to override save_sched_clock_state and restore_sched_clock_state
- * functions of x86_platform. The Hyper-V clock counter is reset during
- * suspend-resume and the offset used to measure time needs to be
- * corrected, post resume.
- */
-static void hv_save_sched_clock_state(void)
-{
-	old_save_sched_clock_state();
-	save_hv_clock_tsc_state();
-}
-
-static void hv_restore_sched_clock_state(void)
-{
-	restore_hv_clock_tsc_state();
-	old_restore_sched_clock_state();
-}
-
-static void __init x86_setup_ops_for_tsc_pg_clock(void)
-{
-	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
-		return;
-
-	old_save_sched_clock_state = x86_platform.save_sched_clock_state;
-	x86_platform.save_sched_clock_state = hv_save_sched_clock_state;
-
-	old_restore_sched_clock_state = x86_platform.restore_sched_clock_state;
-	x86_platform.restore_sched_clock_state = hv_restore_sched_clock_state;
-}
 #endif /* CONFIG_HYPERV */
 
 static uint32_t  __init ms_hyperv_platform(void)
@@ -635,7 +578,6 @@ static void __init ms_hyperv_init_platform(void)
 
 	/* Register Hyper-V specific clocksource */
 	hv_init_clocksource();
-	x86_setup_ops_for_tsc_pg_clock();
 	hv_vtl_init_platform();
 #endif
 	/*
diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index f00019b078a7..86a55167bf5d 100644
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -534,10 +534,60 @@ static __always_inline void hv_setup_sched_clock(void *sched_clock)
 	sched_clock_register(sched_clock, 64, NSEC_PER_SEC);
 }
 #elif defined CONFIG_PARAVIRT
+static u64 hv_ref_counter_at_suspend;
+static void (*old_save_sched_clock_state)(void);
+static void (*old_restore_sched_clock_state)(void);
+
+/*
+ * Hyper-V clock counter resets during hibernation. Save and restore clock
+ * offset during suspend/resume, while also considering the time passed
+ * before suspend. This is to make sure that sched_clock using hv tsc page
+ * based clocksource, proceeds from where it left off during suspend and
+ * it shows correct time for the timestamps of kernel messages after resume.
+ */
+static void save_hv_clock_tsc_state(void)
+{
+	hv_ref_counter_at_suspend = hv_read_reference_counter();
+}
+
+static void restore_hv_clock_tsc_state(void)
+{
+	/*
+	 * Adjust the offsets used by hv tsc clocksource to
+	 * account for the time spent before hibernation.
+	 * adjusted value = reference counter (time) at suspend
+	 *                - reference counter (time) now.
+	 */
+	hv_adj_sched_clock_offset(hv_ref_counter_at_suspend - hv_read_reference_counter());
+}
+/*
+ * Functions to override save_sched_clock_state and restore_sched_clock_state
+ * functions of x86_platform. The Hyper-V clock counter is reset during
+ * suspend-resume and the offset used to measure time needs to be
+ * corrected, post resume.
+ */
+static void hv_save_sched_clock_state(void)
+{
+	old_save_sched_clock_state();
+	save_hv_clock_tsc_state();
+}
+
+static void hv_restore_sched_clock_state(void)
+{
+	restore_hv_clock_tsc_state();
+	old_restore_sched_clock_state();
+}
+
 static __always_inline void hv_setup_sched_clock(void *sched_clock)
 {
 	/* We're on x86/x64 *and* using PV ops */
 	paravirt_set_sched_clock(sched_clock);
+
+	old_save_sched_clock_state = x86_platform.save_sched_clock_state;
+	x86_platform.save_sched_clock_state = hv_save_sched_clock_state;
+
+	old_restore_sched_clock_state = x86_platform.restore_sched_clock_state;
+	x86_platform.restore_sched_clock_state = hv_restore_sched_clock_state;
 }
 #else /* !CONFIG_GENERIC_SCHED_CLOCK && !CONFIG_PARAVIRT */
 static __always_inline void hv_setup_sched_clock(void *sched_clock) {}
-- 
2.48.1.711.g2feabab25a-goog