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

diff --git a/arch/x86/include/asm/vdso/gettimeofday.h b/arch/x86/include/asm/vdso/gettimeofday.h
index 0badf0a..c81858d 100644 (file)
--- a/arch/x86/include/asm/vdso/gettimeofday.h
+++ b/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
 

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ceb7c5e..99d97ba 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/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) &

diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index bcd9042..c643bfe 100644 (file)
--- a/drivers/clocksource/hyperv_timer.c
+++ b/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)

diff --git a/include/clocksource/hyperv_timer.h b/include/clocksource/hyperv_timer.h
index 536f897..6cdc873 100644 (file)
--- a/include/clocksource/hyperv_timer.h
+++ b/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; }