Messages in this thread | | | From | Peter Hilber <> | Subject | [RFC PATCH 0/7] Add virtio_rtc module and related changes | Date | Fri, 30 Jun 2023 19:10:43 +0200 |
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This patch series adds the virtio_rtc module, and related bugfixes and small interface extensions. 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.
This patch series depends, through its arm_arch_timer patch, on patch "arm64/arch_timer: Provide noinstr sched_clock_read() functions" [3] which seems not to be available with all development branches yet. Pull [2] for an equivalent series on top of mst/linux-next, which should apply without the above patch. Pull [5] for an equivalent series on top of tip/timers/core. Pull [6] for an equivalent series on top of tip/sched/core.
The series first fixes some bugs in the get_device_system_crosststamp() interpolation code, which is required for reliable virtio_rtc operation. Next, expose some Arm Generic Timer clocksource details for virtio_rtc. Last, add the virtio_rtc implementation.
For the Virtio RTC device, there is currently a proprietary implementation, which has been used for provisional testing.
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 [4] 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.
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
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 UTC", 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
This patch series adds virtio_rtc not in drivers/ptp, but as a generic Virtio driver. In the near future, virtio_rtc should be extended with an RTC Class driver, along with extensions to the Virtio RTC device draft spec to support RTC alarms.
Feedback is greatly appreciated.
[1] https://lists.oasis-open.org/archives/virtio-comment/202306/msg00592.html [2] https://github.com/OpenSynergy/linux virtio-rtc-v1-on-mst-vhost-linux-next [3] https://lore.kernel.org/r/20230519102715.435618812@infradead.org [4] https://chrony.tuxfamily.org/ [5] https://github.com/OpenSynergy/linux virtio-rtc-v1-on-tip-timers-core [6] https://github.com/OpenSynergy/linux virtio-rtc-v1-on-tip-sched-core
Peter Hilber (7): timekeeping: Fix cross-timestamp interpolation on counter wrap timekeeping: Fix cross-timestamp interpolation corner case decision timekeeping: Fix cross-timestamp interpolation for non-x86 clocksource: arm_arch_timer: Export counter type, clocksource virtio_rtc: Add module and driver core virtio_rtc: Add PTP clocks virtio_rtc: Add Arm Generic Timer cross-timestamping
MAINTAINERS | 7 + drivers/clocksource/arm_arch_timer.c | 16 + drivers/virtio/Kconfig | 43 ++ drivers/virtio/Makefile | 4 + drivers/virtio/virtio_rtc_arm.c | 44 ++ drivers/virtio/virtio_rtc_driver.c | 841 +++++++++++++++++++++++++++ drivers/virtio/virtio_rtc_internal.h | 85 +++ drivers/virtio/virtio_rtc_ptp.c | 384 ++++++++++++ include/clocksource/arm_arch_timer.h | 19 + include/uapi/linux/virtio_rtc.h | 159 +++++ kernel/time/timekeeping.c | 20 +- 11 files changed, 1615 insertions(+), 7 deletions(-) create mode 100644 drivers/virtio/virtio_rtc_arm.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: 6352a698ca5bf26a9199202666b16cf741f579f6 -- 2.39.2
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