[PATCH v2 10/13] clocksource: hyper-v: Provide noinstr sched_clock()

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

With the intent to provide local_clock_noinstr(), a variant of
local_clock() that's safe to be called from noinstr code (with the
assumption that any such code will already be non-preemptible),
prepare for things by making the Hyper-V TSC and MSR sched_clock
implementations noinstr.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Co-developed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/mshyperv.h    |    5 +++++
 drivers/clocksource/hyperv_timer.c |   32 ++++++++++++++++++--------------
 2 files changed, 23 insertions(+), 14 deletions(-)

--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -257,6 +257,11 @@ void hv_set_register(unsigned int reg, u
 u64 hv_get_non_nested_register(unsigned int reg);
 void hv_set_non_nested_register(unsigned int reg, u64 value);
 
+static __always_inline u64 hv_raw_get_register(unsigned int reg)
+{
+	return __rdmsr(reg);
+}
+
 #else /* CONFIG_HYPERV */
 static inline void hyperv_init(void) {}
 static inline void hyperv_setup_mmu_ops(void) {}
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -365,6 +365,20 @@ void hv_stimer_global_cleanup(void)
 }
 EXPORT_SYMBOL_GPL(hv_stimer_global_cleanup);
 
+static __always_inline u64 read_hv_clock_msr(void)
+{
+	/*
+	 * Read the partition counter to get the current tick count. This count
+	 * is set to 0 when the partition is created and is incremented in 100
+	 * nanosecond units.
+	 *
+	 * Use hv_raw_get_register() because this function is used from
+	 * noinstr. Notable; while HV_REGISTER_TIME_REF_COUNT is a synthetic
+	 * register it doesn't need the GHCB path.
+	 */
+	return hv_raw_get_register(HV_REGISTER_TIME_REF_COUNT);
+}
+
 /*
  * Code and definitions for the Hyper-V clocksources.  Two
  * clocksources are defined: one that reads the Hyper-V defined MSR, and
@@ -393,7 +407,7 @@ struct ms_hyperv_tsc_page *hv_get_tsc_pa
 }
 EXPORT_SYMBOL_GPL(hv_get_tsc_page);
 
-static notrace u64 read_hv_clock_tsc(void)
+static __always_inline u64 read_hv_clock_tsc(void)
 {
 	u64 cur_tsc, time;
 
@@ -404,7 +418,7 @@ static notrace u64 read_hv_clock_tsc(voi
 	 * to the MSR in case the TSC page indicates unavailability.
 	 */
 	if (!hv_read_tsc_page_tsc(tsc_page, &cur_tsc, &time))
-		time = hv_get_register(HV_REGISTER_TIME_REF_COUNT);
+		time = read_hv_clock_msr();
 
 	return time;
 }
@@ -414,7 +428,7 @@ static u64 notrace read_hv_clock_tsc_cs(
 	return read_hv_clock_tsc();
 }
 
-static u64 notrace read_hv_sched_clock_tsc(void)
+static u64 noinstr read_hv_sched_clock_tsc(void)
 {
 	return (read_hv_clock_tsc() - hv_sched_clock_offset) *
 		(NSEC_PER_SEC / HV_CLOCK_HZ);
@@ -466,22 +480,12 @@ static struct clocksource hyperv_cs_tsc
 #endif
 };
 
-static u64 notrace read_hv_clock_msr(void)
-{
-	/*
-	 * Read the partition counter to get the current tick count. This count
-	 * is set to 0 when the partition is created and is incremented in
-	 * 100 nanosecond units.
-	 */
-	return hv_get_register(HV_REGISTER_TIME_REF_COUNT);
-}
-
 static u64 notrace read_hv_clock_msr_cs(struct clocksource *arg)
 {
 	return read_hv_clock_msr();
 }
 
-static u64 notrace read_hv_sched_clock_msr(void)
+static u64 noinstr read_hv_sched_clock_msr(void)
 {
 	return (read_hv_clock_msr() - hv_sched_clock_offset) *
 		(NSEC_PER_SEC / HV_CLOCK_HZ);