On 11.03.24 20:46, Alexandre Belloni wrote: > On 11/03/2024 19:28:50+0100, Peter Hilber wrote: >>>> Perhaps this might be handled by the driver also setting a virtio-rtc >>>> monotonic alarm (which uses a clock similar to CLOCK_BOOTTIME_ALARM). >>>> The >>>> virtio-rtc monotonic alarm would just be used to wake up in case it was >>>> a >>>> CLOCK_BOOTTIME_ALARM alarm. >>>> >>>> Otherwise, the behavior should not differ from other RTC class drivers. >>>> >>> >>> What I don't quite get is how this is actually related to RTCs. This >>> would be a super imprecise mechanism to get the current time and date >>> from the host to the guest which is what I think your are trying to do, >>> especially since this is not supporting UIE. >>> The host system clock may come from reading the RTC at some point in >>> time but more likely from another source so is it really the best >>> synchronization mechanism? >> >> Hello, >> >> thank you for your comments. >> >> The main motivation to have the RTC class driver is the RTC alarm >> (discussed below). >> >> As for synchronization, virtio_rtc also offers a PTP clock [1] which will >> be more precise, but which needs a user space daemon. As for RTC-based >> initial synchronization, my idea was to propose, in a second step, an >> optional op for rtc_class_ops, which would read the clock with nanosecond >> precision. This optional op could then be used in rtc_hctosys(), so there >> would be no need for UIE waiting. > > This would be a clear NAK, rtc_hctosys should use UIE to have proper > synchronisation. It currently does a very bad job reading the RTC and it > is a pity it has been mandated by systemd as useerspace is definitively > better placed to set the system time. I'm still very tempted delaying > everyone's boot by one second and make rtc_hctosys precise for all the > supported HW and not just a single driver. > OK. I plan to add a PPS feature to virtio_rtc so that it can support UIE. AFAIU this is not required for the initial driver version. Thanks for the comments, Peter [...]
