timekeeping: Add clocksource read_snapshot() method and hw_cycles to snapshot

Add a read_snapshot() callback to struct clocksource which returns the
derived clocksource value while also providing the underlying hardware
counter reading and the related clocksource ID.

This allows ktime_get_snapshot_id() to populate new hw_cycles and hw_csid
fields in struct system_time_snapshot.

For clocksources that are derived from an underlying counter (e.g., Hyper-V
TSC page scales TSC to 10MHz, kvmclock scales TSC to 1GHz), this provides
atomic access to both the derived value needed for timekeeping
calculations, and the raw hardware counter needed by consumers like KVM's
master clock and the vmclock PTP driver.

[ tglx: Reworked it slightly ]

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Assisted-by: Kiro:claude-opus-4.6-1m
Link: https://patch.msgid.link/20260526230635.136914-1-dwmw2@infradead.org
Link: https://patch.msgid.link/20260529195558.202568489@kernel.org

1 files changed, 20 insertions, 1 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 65d870c2d40d..0d5b67f609bb 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -320,6 +320,7 @@ static __always_inline u64 tk_clock_read(const struct tk_read_base *tkr)
 
 	return clock->read(clock);
 }
+
 static inline void clocksource_disable_inline_read(void) { }
 static inline void clocksource_enable_inline_read(void) { }
 #endif
@@ -1187,6 +1188,18 @@ noinstr time64_t __ktime_get_real_seconds(void)
 	return tk->xtime_sec;
 }
 
+static inline u64 tk_clock_read_snapshot(const struct tk_read_base *tkr,
+					 struct clocksource_hw_snapshot *chs)
+{
+	struct clocksource *clock = READ_ONCE(tkr->clock);
+
+	if (unlikely(clock->read_snapshot))
+		return clock->read_snapshot(clock, chs);
+
+	return clock->read(clock);
+}
+
+
 /**
  * ktime_get_snapshot_id -  Simultaneously snapshot a given clock ID with
  *			    CLOCK_MONOTONIC_RAW and the underlying
@@ -1237,14 +1250,20 @@ void ktime_get_snapshot_id(clockid_t clock_id, struct system_time_snapshot *syst
 	tk = &tkd->timekeeper;
 
 	do {
+		struct clocksource_hw_snapshot chs = { };
+
 		seq = read_seqcount_begin(&tkd->seq);
 
 		/* Aux clocks can be invalid */
 		if (!tk->clock_valid)
 			return;
 
-		now = tk_clock_read(&tk->tkr_mono);
+		now = tk_clock_read_snapshot(&tk->tkr_mono, &chs);
 		systime_snapshot->cs_id = tk->tkr_mono.clock->id;
+
+		systime_snapshot->hw_cycles = chs.hw_cycles;
+		systime_snapshot->hw_csid = chs.hw_csid;
+
 		systime_snapshot->cs_was_changed_seq = tk->cs_was_changed_seq;
 		systime_snapshot->clock_was_set_seq = tk->clock_was_set_seq;