netdev - Re: [PATCH v4 0/7] ptp: vmclock: Add VM generation counter and ACPI notification

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Message-ID: <5fe241dd-ae17-4cfe-80f7-e7ea80f8b781@redhat.com>
Date: Sun, 28 Dec 2025 15:42:32 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: "Chalios, Babis" <bchalios@...zon.es>, "robh@...nel.org"
 <robh@...nel.org>, "krzk+dt@...nel.org" <krzk+dt@...nel.org>,
 "conor+dt@...nel.org" <conor+dt@...nel.org>,
 "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>
Cc: "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Graf (AWS), Alexander" <graf@...zon.de>,
 "mzxreary@...inter.de" <mzxreary@...inter.de>,
 "Cali, Marco" <xmarcalx@...zon.co.uk>
Subject: Re: [PATCH v4 0/7] ptp: vmclock: Add VM generation counter and ACPI
 notification

On 12/18/25 3:24 PM, Chalios, Babis wrote:
> 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. Implementing
> the notification is optional for the device. VMClock device will declare
> that it implements the notification by setting
> VMCLOCK_FLAG_NOTIFICATION_PRESENT bit in vmclock_abi flags. Hypervisors
> that implement the notification must send an ACPI notification every
> time seq_count changes to an even number. The driver will propagate
> these notifications to userspace via the poll() interface.

There are a few formal things to adjust (reverse christmas tree order in
variable declaration, empty changelog in patch 6/7, prefer #if
IS_ENABLED() over #ifdef). Moreover:

## Form letter - net-next-closed

The net-next tree is closed for new drivers, features, code refactoring
and optimizations due to the merge window and the winter break. We are
currently accepting bug fixes only.

Please repost when net-next reopens after Jan 2nd.

RFC patches sent for review only are obviously welcome at any time.