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ptp: vmclock: Use hw_cycles from snapshot for precise TSC pairing

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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
This commit is contained in:
David Woodhouse
2026-06-04 10:35:18 +01:00
committed by Thomas Gleixner
parent 19fa3e5064
commit bc484a5096
1 changed files with 4 additions and 0 deletions
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4
drivers/ptp/ptp_vmclock.c
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@@ -140,6 +140,10 @@ static int vmclock_get_crosststamp(struct vmclock_state *st,
if (sts->pre_sts.cs_id == st->cs_id) {
cycle = sts->pre_sts.cycles;
sts->post_sts = sts->pre_sts;
} else if (sts->pre_sts.hw_csid == st->cs_id &&
sts->pre_sts.hw_cycles) {
cycle = sts->pre_sts.hw_cycles;
sts->post_sts = sts->pre_sts;
} else {
cycle = get_cycles();
ptp_read_system_postts(sts);