linux-kernel - Re: [PATCH v3 0/4] ptp: vmclock: Add VM generation counter and ACPI notification

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Message-ID: <de13a729-f590-44e8-8177-68bc43809048@redhat.com>
Date: Thu, 4 Dec 2025 12:09:10 +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 v3 0/4] ptp: vmclock: Add VM generation counter and ACPI
 notification

On 12/3/25 1:35 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.

Linux tagged 6.18 final, so net-next is closed for new code submissions
per the announcement at
https://lore.kernel.org/20251130174502.3908e3ee@kernel.org

/P