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Message-ID: <ucphq5pdqdv35anjmrud5egltbbgfwdourk3ahchckk6iwpfmc@goteqcewulgq>
Date: Tue, 25 Feb 2025 13:34:51 +0100
From: Peter Hilber <quic_philber@...cinc.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
CC: Jason Wang <jasowang@...hat.com>, Trilok Soni <quic_tsoni@...cinc.com>,
Srivatsa Vaddagiri <quic_svaddagi@...cinc.com>,
Eugenio Pérez
<eperezma@...hat.com>,
Richard Cochran <richardcochran@...il.com>,
Xuan Zhuo
<xuanzhuo@...ux.alibaba.com>,
<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
<virtualization@...ts.linux.dev>,
David Woodhouse
<dwmw2@...radead.org>,
"Ridoux, Julien" <ridouxj@...zon.com>, Marc Zyngier
<maz@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Daniel Lezcano
<daniel.lezcano@...aro.org>,
Alexandre Belloni
<alexandre.belloni@...tlin.com>,
Parav Pandit <parav@...dia.com>,
"Matias
Ezequiel Vara Larsen" <mvaralar@...hat.com>,
Cornelia Huck
<cohuck@...hat.com>, <virtio-dev@...ts.linux.dev>,
<linux-arm-kernel@...ts.infradead.org>, <linux-rtc@...r.kernel.org>
Subject: Re: [PATCH v5 0/4] Add virtio_rtc module
On Tue, Feb 25, 2025 at 07:18:59AM -0500, Michael S. Tsirkin wrote:
> On Wed, Feb 19, 2025 at 08:32:55PM +0100, Peter Hilber wrote:
> > This series implements a driver for a virtio-rtc device conforming to spec
> > proposal v7 [1]. It includes a PTP clock driver and an RTC class driver
> > with alarm.
> >
> > v5 updates
> > ==========
> >
> > Important changes compared to the previous driver series [3] are:
> >
> > - Update to spec proposal v7 [1].
> >
> > - Fix multiple initialization related bugs.
> >
> > - Drop the RFC tag, since no major outstanding issues are apparent.
> >
> > Overview
> > ========
> >
> > 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. The driver can read the clocks with simple
> > or more accurate methods. Optionally, the driver can set an alarm.
> >
> > For the Virtio RTC device, there is currently a proprietary implementation,
> > which has been used for testing.
> >
> > PTP clock interface
> > ===================
> >
> > 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 the
> > 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 clock
> > (from the Virtio RTC device). This works even when the Virtio device
> > response is slow compared to ptp_kvm hypercalls.
> >
> > The following illustrates a test using PTP_SYS_OFFSET_PRECISE, with
> > interspersed strace log and chrony [2] refclocks log, on arm64. In the
> > example, chrony tracks a virtio_rtc PTP clock ("PHCV", /dev/ptp0). The raw
> > offset between the virtio_rtc clock and CLOCK_REALTIME is 0 to 1 ns. At the
> > device side, the Virtio RTC device artificially delays both the clock read
> > request, and the response, by 50 ms. Cross-timestamp interpolation still
> > works with this delay. chrony also monitors a ptp_kvm clock ("PHCK",
> > /dev/ptp3) for comparison, which yields a similar offset.
> >
> > ioctl(5</dev/ptp3>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.000329>
> > ===============================================================================
> > Date (UTC) Time Refid DP L P Raw offset Cooked offset Disp.
> > ===============================================================================
> > 2023-06-29 18:49:55.595742 PHCK 0 N 0 1.000000e-09 8.717931e-10 5.500e-08
> > 2023-06-29 18:49:55.595742 PHCK - N - - 8.717931e-10 5.500e-08
> > ioctl(6</dev/ptp0>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.101545>
> > 2023-06-29 18:49:56.147766 PHCV 0 N 0 1.000000e-09 8.801870e-10 5.500e-08
> > 2023-06-29 18:49:56.147766 PHCV - N - - 8.801870e-10 5.500e-08
> > ioctl(5</dev/ptp3>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.000195>
> > 2023-06-29 18:49:56.202446 PHCK 0 N 0 1.000000e-09 7.364180e-10 5.500e-08
> > 2023-06-29 18:49:56.202446 PHCK - N - - 7.364180e-10 5.500e-08
> > ioctl(6</dev/ptp0>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.101484>
> > 2023-06-29 18:49:56.754641 PHCV 0 N 0 0.000000e+00 -2.617368e-10 5.500e-08
> > 2023-06-29 18:49:56.754641 PHCV - N - - -2.617368e-10 5.500e-08
> > ioctl(5</dev/ptp3>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.000270>
> > 2023-06-29 18:49:56.809282 PHCK 0 N 0 1.000000e-09 7.779321e-10 5.500e-08
> > 2023-06-29 18:49:56.809282 PHCK - N - - 7.779321e-10 5.500e-08
> > ioctl(6</dev/ptp0>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.101510>
> > 2023-06-29 18:49:57.361376 PHCV 0 N 0 0.000000e+00 -2.198794e-10 5.500e-08
> > 2023-06-29 18:49:57.361376 PHCV - N - - -2.198794e-10 5.500e-08
> >
> > This patch series only adds special support for the Arm Generic Timer
> > clocksource. At the driver side, it should be easy to support more
> > clocksources.
> >
> > Fallback PTP clock interface
> > ----------------------------
> >
> > Without special support for the current clocksource, time synchronization
> > programs can still use ioctl PTP_SYS_OFFSET_EXTENDED2 aka
> > PTP_SYS_OFFSET_EXTENDED. In this case, precision will generally be worse
> > and will depend on the Virtio device response characteristics.
> >
> > The following illustrates a test using PTP_SYS_OFFSET_EXTENDED, with
> > interspersed strace log and chrony refclocks log, on x86-64 (with `ts'
> > values omitted):
> >
> > ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> > ===============================================================================
> > Date (UTC) Time Refid DP L P Raw offset Cooked offset Disp.
> > ===============================================================================
> > 2023-06-28 14:11:26.697782 PHCV 0 N 0 3.318200e-05 3.450891e-05 4.611e-06
> > 2023-06-28 14:11:26.697782 PHCV - N - - 3.450891e-05 4.611e-06
> > ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> > 2023-06-28 14:11:27.208763 PHCV 0 N 0 -3.792800e-05 -4.023965e-05 4.611e-06
> > 2023-06-28 14:11:27.208763 PHCV - N - - -4.023965e-05 4.611e-06
> > ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> > 2023-06-28 14:11:27.722818 PHCV 0 N 0 -3.328600e-05 -3.134404e-05 4.611e-06
> > 2023-06-28 14:11:27.722818 PHCV - N - - -3.134404e-05 4.611e-06
> > ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> > 2023-06-28 14:11:28.233572 PHCV 0 N 0 -4.966900e-05 -4.584331e-05 4.611e-06
> > 2023-06-28 14:11:28.233572 PHCV - N - - -4.584331e-05 4.611e-06
> > ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> > 2023-06-28 14:11:28.742737 PHCV 0 N 0 4.902700e-05 5.361388e-05 4.611e-06
> > 2023-06-28 14:11:28.742737 PHCV - N - - 5.361388e-05 4.611e-06
> >
> > PTP clock setup
> > ---------------
> >
> > The following udev rule can be used to get a symlink /dev/ptp_virtio to the
> > UTC clock:
> >
> > SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 0, variant 0", SYMLINK += "ptp_virtio"
> >
> > The following chrony configuration directive can then be added in
> > /etc/chrony/chrony.conf to synchronize to the Virtio UTC clock:
> >
> > refclock PHC /dev/ptp_virtio refid PHCV poll -1 dpoll -1
> >
> > RTC interface
> > =============
> >
> > This patch series adds virtio_rtc as a generic Virtio driver, including
> > both a PTP clock driver and an RTC class driver.
> >
> > Feedback is greatly appreciated.
> >
> > [1] https://lore.kernel.org/virtio-comment/20250123101616.664-1-quic_philber@quicinc.com/
> > [2] https://chrony.tuxfamily.org/
> > [3] https://lore.kernel.org/lkml/20241219201118.2233-1-quic_philber@quicinc.com/
> >
> > Changelog
> > =========
> >
> > v5:
> >
> > - Update to virtio-rtc spec v7, essentially removing definitions.
> >
> > - Fix multiple bugs after readying device during probe and restore.
> >
> > - Actually initialize Virtio clock id for RTC class device.
> >
> > - Add freeze/restore ops already in first patch.
> >
> > - Minor changes:
> >
> > - Use new APIs devm_device_init_wakeup(), secs_to_jiffies().
> >
> > - Fix style issues.
> >
> > - Improve logging types and clarity.
> >
> > - Drop unnecessary memory barrier pair.
> >
> > - Return error status from device, whenever available.
> >
> > v4:
> >
> > - Update Virtio interface to spec v6.
> >
> > - Distinguish UTC-like clocks by handling of leap seconds (spec v6).
> >
> > - Do not create RTC class device for clocks which may step on leap seconds
> > (Alexandre Belloni).
> >
> > - Clear RTC class feature bit instead of defining reduced ops
> > (Alexandre Belloni).
> >
> > - For PTP clock name, always use numeric clock type, and numeric variant.
> >
> > - Use macros for 64-bit divisions.
> >
> > - Remove unnecessary memory barriers.
> >
> > - Cosmetic improvements.
> >
> > - Drop upstreamed timekeeping bugfixes from series.
> >
> > v3:
> >
> > - Update to conform to virtio spec RFC v3 (no significant behavioral
> > changes).
> >
> > - Add RTC class driver with alarm according to virtio spec RFC v3.
> >
> > - For cross-timestamp corner case fix, switch back to v1 style closed
> > interval test (Thomas Gleixner).
> >
> > v2:
> >
> > - Depend on patch series "treewide: Use clocksource id for
> > get_device_system_crosststamp()" to avoid requiring a clocksource pointer
> > with get_device_system_crosststamp().
> >
> > - Assume Arm Generic Timer will use CP15 virtual counter. Drop
> > arm_arch_timer helper functions (Marc Zyngier).
> >
> > - Improve cross-timestamp fixes problem description and implementation
> > (John Stultz).
> >
> >
> > Peter Hilber (4):
> > virtio_rtc: Add module and driver core
> > virtio_rtc: Add PTP clocks
> > virtio_rtc: Add Arm Generic Timer cross-timestamping
> > virtio_rtc: Add RTC class driver
> >
> > MAINTAINERS | 7 +
> > drivers/virtio/Kconfig | 64 ++
> > drivers/virtio/Makefile | 5 +
> > drivers/virtio/virtio_rtc_arm.c | 23 +
> > drivers/virtio/virtio_rtc_class.c | 262 +++++
> > drivers/virtio/virtio_rtc_driver.c | 1404 ++++++++++++++++++++++++++
> > drivers/virtio/virtio_rtc_internal.h | 122 +++
> > drivers/virtio/virtio_rtc_ptp.c | 347 +++++++
> > include/uapi/linux/virtio_rtc.h | 237 +++++
>
>
> Also, should this driver live under
> ./drivers/rtc/virtio/
>
> ?
It has a PTP clock driver as well, so I think drivers/rtc would not be
the best location. I tried to avoid splitting the driver across
subsystems.
Maybe it could live under drivers/virtio/rtc, if you don't like having 5
new files in the directory?
Thanks for the comment,
Peter
>
>
>
> > 9 files changed, 2471 insertions(+)
> > create mode 100644 drivers/virtio/virtio_rtc_arm.c
> > create mode 100644 drivers/virtio/virtio_rtc_class.c
> > create mode 100644 drivers/virtio/virtio_rtc_driver.c
> > create mode 100644 drivers/virtio/virtio_rtc_internal.h
> > create mode 100644 drivers/virtio/virtio_rtc_ptp.c
> > create mode 100644 include/uapi/linux/virtio_rtc.h
> >
> >
> > base-commit: 8936cec5cb6e27649b86fabf383d7ce4113bba49
> > --
> > 2.43.0
>
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