When the system clocksource is kvmclock or Hyper-V (not the TSC directly), vmclock_get_crosststamp() falls through to a separate get_cycles() call, losing the atomic pairing between the system time snapshot and the TSC reading. Now that ktime_get_snapshot_id() populates hw_cycles with the underlying TSC value for derived clocksources, use it when available. This gives a perfect (system_time, tsc) pairing for the device time calculation. The SUPPORT_KVMCLOCK wrapper is still needed to convert the TSC into kvmclock nanoseconds for system_counter->cycles, because otherwise get_device_system_crosststamp() can't interpret the result against the system clock. Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Signed-off-by: Thomas Gleixner <tglx@kernel.org> Assisted-by: Kiro:claude-opus-4.6-1m Link: https://patch.msgid.link/20260604095755.64849-4-dwmw2@infradead.org