clocksource: hyper-v: Adjust hv_read_tsc_page_tsc() to avoid special casing U64_MAX

Currently hv_read_tsc_page_tsc() (ab)uses the (valid) time value of
U64_MAX as an error return. This breaks the clean wrap-around of the
clock.

Modify the function signature to return a boolean state and provide
another u64 pointer to store the actual time on success. This obviates
the need to steal one time value and restores the full counter width.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Michael Kelley <mikelley@microsoft.com>  # Hyper-V
Link: https://lore.kernel.org/r/20230519102715.775630881@infradead.org

arch/x86/include/asm/vdso/gettimeofday.h

@@ -238,10 +238,12 @@ static u64 vread_pvclock(void)
 #ifdef CONFIG_HYPERV_TIMER
 static u64 vread_hvclock(void)
 {
-	u64 ret = hv_read_tsc_page(&hvclock_page);
-	if (likely(ret != U64_MAX))
-		ret &= S64_MAX;
-	return ret;
+	u64 tsc, time;
+
+	if (hv_read_tsc_page_tsc(&hvclock_page, &tsc, &time))
+		return time & S64_MAX;
+
+	return U64_MAX;
 }
 #endif
 

arch/x86/kvm/x86.c

@@ -2799,14 +2799,13 @@ static u64 read_tsc(void)
 static inline u64 vgettsc(struct pvclock_clock *clock, u64 *tsc_timestamp,
 			  int *mode)
 {
-	long v;
 	u64 tsc_pg_val;
+	long v;
 
 	switch (clock->vclock_mode) {
 	case VDSO_CLOCKMODE_HVCLOCK:
-		tsc_pg_val = hv_read_tsc_page_tsc(hv_get_tsc_page(),
-						  tsc_timestamp);
-		if (tsc_pg_val != U64_MAX) {
+		if (hv_read_tsc_page_tsc(hv_get_tsc_page(),
+					 tsc_timestamp, &tsc_pg_val)) {
 			/* TSC page valid */
 			*mode = VDSO_CLOCKMODE_HVCLOCK;
 			v = (tsc_pg_val - clock->cycle_last) &

drivers/clocksource/hyperv_timer.c

@@ -393,14 +393,20 @@ struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
 }
 EXPORT_SYMBOL_GPL(hv_get_tsc_page);
 
-static u64 notrace read_hv_clock_tsc(void)
+static notrace u64 read_hv_clock_tsc(void)
 {
-	u64 current_tick = hv_read_tsc_page(hv_get_tsc_page());
+	u64 cur_tsc, time;
 
-	if (current_tick == U64_MAX)
-		current_tick = hv_get_register(HV_REGISTER_TIME_REF_COUNT);
+	/*
+	 * The Hyper-V Top-Level Function Spec (TLFS), section Timers,
+	 * subsection Refererence Counter, guarantees that the TSC and MSR
+	 * times are in sync and monotonic. Therefore we can fall back
+	 * 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);
 
-	return current_tick;
+	return time;
 }
 
 static u64 notrace read_hv_clock_tsc_cs(struct clocksource *arg)

include/clocksource/hyperv_timer.h

@@ -38,8 +38,9 @@ extern void hv_remap_tsc_clocksource(void);
 extern unsigned long hv_get_tsc_pfn(void);
 extern struct ms_hyperv_tsc_page *hv_get_tsc_page(void);
 
-static inline notrace u64
-hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg, u64 *cur_tsc)
+static __always_inline bool
+hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
+		     u64 *cur_tsc, u64 *time)
 {
 	u64 scale, offset;
 	u32 sequence;
@@ -63,7 +64,7 @@ hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg, u64 *cur_tsc)
 	do {
 		sequence = READ_ONCE(tsc_pg->tsc_sequence);
 		if (!sequence)
-			return U64_MAX;
+			return false;
 		/*
 		 * Make sure we read sequence before we read other values from
 		 * TSC page.
@@ -82,15 +83,8 @@ hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg, u64 *cur_tsc)
 
 	} while (READ_ONCE(tsc_pg->tsc_sequence) != sequence);
 
-	return mul_u64_u64_shr(*cur_tsc, scale, 64) + offset;
-}
-
-static inline notrace u64
-hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)
-{
-	u64 cur_tsc;
-
-	return hv_read_tsc_page_tsc(tsc_pg, &cur_tsc);
+	*time = mul_u64_u64_shr(*cur_tsc, scale, 64) + offset;
+	return true;
 }
 
 #else /* CONFIG_HYPERV_TIMER */
@@ -104,10 +98,10 @@ static inline struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
 	return NULL;
 }
 
-static inline u64 hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
-				       u64 *cur_tsc)
+static __always_inline bool
+hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg, u64 *cur_tsc, u64 *time)
 {
-	return U64_MAX;
+	return false;
 }
 
 static inline int hv_stimer_cleanup(unsigned int cpu) { return 0; }