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, 6 insertions, 0 deletions

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 7f03e1cefcf6..984a866d293b 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -279,18 +279,24 @@ static inline bool ktime_get_aux_ts64(clockid_t id, struct timespec64 *kt) { ret
  * struct system_time_snapshot - Simultaneous time capture of CLOCK_MONOTONIC_RAW,
  *				 a selected CLOCK_* and the clocksource counter value
  * @cycles:		Clocksource counter value to produce the system times
+ * @hw_cycles:		For derived clocksources, the hardware counter value from
+ *			which @cycles was derived
  * @systime:		The system time of the selected CLOCK ID
  * @monoraw:		Monotonic raw system time
  * @cs_id:		Clocksource ID
+ * @hw_csid:		Clocksource ID of the underlying hardware counter for derived
+ *			clocksources which implement the read_snapshot() callback.
  * @clock_was_set_seq:	The sequence number of clock-was-set events
  * @cs_was_changed_seq:	The sequence number of clocksource change events
  * @valid:		True if the snapshot is valid
  */
 struct system_time_snapshot {
 	u64			cycles;
+	u64			hw_cycles;
 	ktime_t			systime;
 	ktime_t			monoraw;
 	enum clocksource_ids	cs_id;
+	enum clocksource_ids	hw_csid;
 	unsigned int		clock_was_set_seq;
 	u8			cs_was_changed_seq;
 	u8			valid;