Re: [PATCH v5 0/4] Add virtio_rtc module

["Michael S. Tsirkin" <mst@redhat.com>]

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/

?



>  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