linux-kernel - [RFC PATCH 1/2] ptp: vmclock: add vm generation counter

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Message-ID: <20251125153830.11487-2-bchalios@amazon.es>
Date: Tue, 25 Nov 2025 15:38:42 +0000
From: "Chalios, Babis" <bchalios@...zon.es>
To: "richardcochran@...il.com" <richardcochran@...il.com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>, "andrew+netdev@...n.ch"
	<andrew+netdev@...n.ch>, "davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: "Chalios, Babis" <bchalios@...zon.es>, "Graf (AWS), Alexander"
	<graf@...zon.de>, "mzxreary@...inter.de" <mzxreary@...inter.de>
Subject: [RFC PATCH 1/2] ptp: vmclock: add vm generation counter

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.

Signed-off-by: Babis Chalios <bchalios@...zon.es>
---
 include/uapi/linux/vmclock-abi.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/uapi/linux/vmclock-abi.h b/include/uapi/linux/vmclock-abi.h
index 2d99b29ac44a..fbf1c5928273 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 +183,19 @@ 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 VM
+	 * is loaded from a snapshot. This event, typically, represents a
+	 * "jump" forward in time. As a result, in this case as well, the
+	 * guest needs to discard any calibrarion against external sources.
+	 * Loading a snapshot in a VM has different semantics than other VM
+	 * events such as live migration, i.e. apart from re-adjusting guest
+	 * clocks a guest user space might want to discard UUIDs, reset
+	 * network connections or reseed entropy, etc. As a result, we
+	 * use a dedicated marker for such events.
+	 */
+	__le64 vm_generation_counter;
 };
 
 #endif /*  __VMCLOCK_ABI_H__ */
-- 
2.34.1