Implement the read_snapshot() callback for the Hyper-V TSC page clock-
source. This returns the derived 10MHz reference time (for timekeeping)
while also providing the raw TSC value that was used to compute it.
When the TSC page is valid, hv_read_tsc_page_tsc() atomically captures both
values from a single RDTSC inside the sequence-counter protected read. When
the TSC page is invalid (sequence == 0), the hw_csid and hw_cycles are set
to zero indicating no value is available.
This enables ktime_get_snapshot_id() to provide the raw TSC to consumers
like KVM's master clock when running nested guests under Hyper-V.
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
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Link: https://patch.msgid.link/20260604095755.64849-2-dwmw2@infradead.org
c51100f9e2