[PATCH v5 1/7] ptp: vmclock: add vm generation counter

Chalios, Babis
2026-01-07T13:26:58Z

Similar to live migration, loading a VM from some saved state (aka
snapshot) is also an event that calls for clock adjustments in the
guest. However, guests might want to take more actions as a response to
such events, e.g. as discarding UUIDs, resetting network connections,
reseeding entropy pools, etc. These are actions that guests don't
typically take during live migration, so add a new field in the
vmclock_abi called vm_generation_counter which informs the guest about
such events.

Hypervisor advertises support for vm_generation_counter through the
VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT flag. Users need to check the
presence of this bit in vmclock_abi flags field before using this flag.

Signed-off-by: Babis Chalios <bchalios@amazon.es>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
include/uapi/linux/vmclock-abi.h | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/include/uapi/linux/vmclock-abi.h b/include/uapi/linux/vmclock-abi.h
index 2d99b29ac44a..937fe00e4f33 100644
--- a/include/uapi/linux/vmclock-abi.h
+++ b/include/uapi/linux/vmclock-abi.h
@@ -115,6 +115,12 @@ struct vmclock_abi {
* bit again after the update, using the about-to-be-valid fields.
*/
#define VMCLOCK_FLAG_TIME_MONOTONIC (1 << 7)
+ /*
+ * If the VM_GEN_COUNTER_PRESENT flag is set, the hypervisor will
+ * bump the vm_generation_counter field every time the guest is
+ * loaded from some save state (restored from a snapshot).
+ */
+#define VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT (1 << 8)

__u8 pad[2];
__u8 clock_status;
@@ -177,6 +183,15 @@ struct vmclock_abi {
__le64 time_frac_sec; /* Units of 1/2^64 of a second */
__le64 time_esterror_nanosec;
__le64 time_maxerror_nanosec;
+
+ /*
+ * This field changes to another non-repeating value when the guest
+ * has been loaded from a snapshot. In addition to handling a
+ * disruption in time (which will also be signalled through the
+ * disruption_marker field), a guest may wish to discard UUIDs,
+ * reset network connections, reseed entropy, etc.
+ */
+ __le64 vm_generation_counter;
};

#endif /* __VMCLOCK_ABI_H__ */
--
2.34.1