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Message-ID: <0e21e3e2be26acd70b5575b9932b3a911c9fe721.camel@infradead.org>
Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes
From: David Woodhouse <dwmw2@infradead.org>
To: Peter Hilber <peter.hilber@opensynergy.com>,
 linux-kernel@vger.kernel.org,  virtualization@lists.linux.dev,
 virtio-dev@lists.oasis-open.org,  linux-arm-kernel@lists.infradead.org,
 linux-rtc@vger.kernel.org
Cc: "Christopher S. Hall" <christopher.s.hall@intel.com>, Jason Wang
 <jasowang@redhat.com>, John Stultz <jstultz@google.com>, "Michael S.
 Tsirkin" <mst@redhat.com>, netdev@vger.kernel.org, Richard Cochran
 <richardcochran@gmail.com>, Stephen Boyd <sboyd@kernel.org>, Thomas
 Gleixner <tglx@linutronix.de>, Xuan Zhuo <xuanzhuo@linux.alibaba.com>, Marc
 Zyngier <maz@kernel.org>, Mark Rutland <mark.rutland@arm.com>, Daniel
 Lezcano <daniel.lezcano@linaro.org>, Alessandro Zummo
 <a.zummo@towertech.it>,  Alexandre Belloni <alexandre.belloni@bootlin.com>,
 "Ridoux, Julien" <ridouxj@amazon.com>
Date: Thu, 07 Mar 2024 14:02:07 +0000
In-Reply-To: <20231218073849.35294-1-peter.hilber@opensynergy.com>
References: <20231218073849.35294-1-peter.hilber@opensynergy.com>
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On Mon, 2023-12-18 at 08:38 +0100, Peter Hilber wrote:
> RFC v3 updates
> --------------
>=20
> This series implements a driver for a virtio-rtc device conforming to spe=
c
> RFC v3 [1]. It now includes an RTC class driver with alarm, in addition t=
o
> the PTP clock driver already present before.
>=20
> This patch series depends on the patch series "treewide: Use clocksource =
id
> for get_device_system_crosststamp()" [3]. Pull [4] to get the combined
> series on top of mainline.
>=20
> Overview
> --------
>=20
> This patch series adds the virtio_rtc module, and related bugfixes. The
> virtio_rtc module implements a driver compatible with the proposed Virtio
> RTC device specification [1]. The Virtio RTC (Real Time Clock) device
> provides information about current time. The device can provide different
> clocks, e.g. for the UTC or TAI time standards, or for physical time
> elapsed since some past epoch.=C2=A0

Hm, should we allow UTC? If you tell me the time in UTC, then
(sometimes) I still don't actually know what the time is, because some
UTC seconds occur twice. UTC only makes sense if you provide the TAI
offset, surely? Should the virtio_rtc specification make it mandatory
to provide such?

Otherwise you're just designing it to allow crappy hypervisors to
expose incomplete information.

> PTP clock interface
> -------------------
>=20
> virtio_rtc exposes clocks as PTP clocks to userspace, similar to ptp_kvm.
> If both the Virtio RTC device and this driver have special support for th=
e
> current clocksource, time synchronization programs can use
> cross-timestamping using ioctl PTP_SYS_OFFSET_PRECISE2 aka
> PTP_SYS_OFFSET_PRECISE. Similar to ptp_kvm, system time synchronization
> with single-digit ns precision is possible with a quiescent reference clo=
ck
> (from the Virtio RTC device). This works even when the Virtio device
> response is slow compared to ptp_kvm hypercalls.

Is PTP the right mechanism for this? As I understand it, PTP is a way
to precisely synchronize one clock with another. But in the case of
virt guests synchronizing against the host, it isn't really *another*
clock. It really is the *same* underlying clock. As the host clock
varies with temperature, for example, so does the guest clock. The only
difference is an offset and (on x86 perhaps) a mathematical scaling of
the frequency.

I was looking at this another way, when I came across this virtio-rtc
work.

My idea was just for the hypervisor to expose its own timekeeping
information =E2=80=94 the counter/TSC value and TAI time at a given moment,
frequency of the counter, and the precision of both that frequency
(=C2=B1PPM) and the TAI timestamp (=C2=B1=C2=B5s).

By putting that in a host/guest shared data structure with a seqcount
for lockless updates, we can update it as time synchronization on the
host is refined, and we can even cleanly handle live migration where
the guest ends up on a completely different host. It allows for use
cases which *really* care (e.g. timestamping financial transactions) to
ensure that there is never even a moment of getting *wrong* timestamps
if they haven't yet resynced after a migration.

Now I'm trying to work out if I should attempt to reconcile with your
existing virtio-rtc work, or just decide that virtio-rtc isn't trying
to solve the actual problem that we have, and go ahead with something
different... ?


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