linux-kernel - [RFC PATCH 0/2] ptp: vmclock: Add VM generation counter and ACPI notification

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Message-ID: <20251125153830.11487-1-bchalios@amazon.es>
Date: Tue, 25 Nov 2025 15:38:31 +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 0/2] ptp: vmclock: Add VM generation counter and ACPI
 notification

Similarly to live migration, starting a VM from some serialized state
(aka snapshot) is an event which calls for adjusting guest clocks, hence
a hypervisor should increase the disruption_marker before resuming the
VM vCPUs, letting the guest know.

However, loading a snapshot, is slightly different than live migration,
especially since we can start multiple VMs from the same serialized
state. Apart from adjusting clocks, the guest needs to take additional
action during such events, e.g. recreate UUIDs, reset network
adapters/connections, reseed entropy pools, etc. These actions are not
necessary during live migration. This calls for a differentiation
between the two triggering events.

We differentiate between the two events via an extra field in the
vmclock_abi, called vm_generation_counter. Whereas hypervisors should
increase the disruption marker in both cases, they should only increase
vm_generation_counter when a snapshot is loaded in a VM (not during live
migration).

Additionally, we attach an ACPI notification to VMClock. Hypervisors
should execute an ACPI notify operation every time seq_count changes to
an even number. We implement the poll() interface for VMClock device.
listeners of poll() will be notified about changes in the ABI every time
the hypervisor sends us a notification.

Babis Chalios (2):
  ptp: vmclock: add vm generation counter
  ptp: vmclock: support device notifications

 drivers/ptp/ptp_vmclock.c        | 85 ++++++++++++++++++++++++++++++--
 include/uapi/linux/vmclock-abi.h | 19 +++++++
 2 files changed, 99 insertions(+), 5 deletions(-)

-- 
2.34.1