netdev - Re: [PATCH net-next v7] ptp: Add support for the AMZNC10C 'vmclock' device

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Message-ID: <ZwQHtD5lVNuc4aAf@hoboy.vegasvil.org>
Date: Mon, 7 Oct 2024 09:09:24 -0700
From: Richard Cochran <richardcochran@...il.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Peter Hilber <peter.hilber@...nsynergy.com>,
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-rtc@...r.kernel.org,
	"Ridoux, Julien" <ridouxj@...zon.com>, virtio-dev@...ts.linux.dev,
	"Luu, Ryan" <rluu@...zon.com>,
	"Chashper, David" <chashper@...zon.com>,
	"Mohamed Abuelfotoh, Hazem" <abuehaze@...zon.com>,
	Paolo Abeni <pabeni@...hat.com>,
	"Christopher S . Hall" <christopher.s.hall@...el.com>,
	Jason Wang <jasowang@...hat.com>, John Stultz <jstultz@...gle.com>,
	"Michael S . Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
	Stephen Boyd <sboyd@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	qemu-devel <qemu-devel@...gnu.org>, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next v7] ptp: Add support for the AMZNC10C 'vmclock'
 device

On Sun, Oct 06, 2024 at 08:17:58AM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> The vmclock device addresses the problem of live migration with
> precision clocks. The tolerances of a hardware counter (e.g. TSC) are
> typically around ±50PPM. A guest will use NTP/PTP/PPS to discipline that
> counter against an external source of 'real' time, and track the precise
> frequency of the counter as it changes with environmental conditions.
> 
> When a guest is live migrated, anything it knows about the frequency of
> the underlying counter becomes invalid. It may move from a host where
> the counter running at -50PPM of its nominal frequency, to a host where
> it runs at +50PPM. There will also be a step change in the value of the
> counter, as the correctness of its absolute value at migration is
> limited by the accuracy of the source and destination host's time
> synchronization.
> 
> In its simplest form, the device merely advertises a 'disruption_marker'
> which indicates that the guest should throw away any NTP synchronization
> it thinks it has, and start again.
> 
> Because the shared memory region can be exposed all the way to userspace
> through the /dev/vmclock0 node, applications can still use time from a
> fast vDSO 'system call', and check the disruption marker to be sure that
> their timestamp is indeed truthful.
> 
> The structure also allows for the precise time, as known by the host, to
> be exposed directly to guests so that they don't have to wait for NTP to
> resync from scratch. The PTP driver consumes this information if present.
> Like the KVM PTP clock, this PTP driver can convert TSC-based cross
> timestamps into KVM clock values. Unlike the KVM PTP clock, it does so
> only when such is actually helpful.
> 
> The values and fields are based on the nascent virtio-rtc specification,
> and the intent is that a version (hopefully precisely this version) of
> this structure will be included as an optional part of that spec. In the
> meantime, this driver supports the simple ACPI form of the device which
> is being shipped in certain commercial hypervisors (and submitted for
> inclusion in QEMU).
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>

Acked-by: Richard Cochran <richardcochran@...il.com>