> From: Paul Irofti <p...@irofti.net> > Date: Sat, 11 Jul 2020 13:50:37 +0300 > > On 2020-07-11 13:46, Mark Kettenis wrote: > >> From: Paul Irofti <p...@irofti.net> > >> Date: Sat, 11 Jul 2020 13:32:22 +0300 > >> > >> Hi, > >> > >> Getting lots of messages about people loving the new timekeep > >> functionality, which I am very happy about, but also some that have the > >> skew too large for it to be enabled. > >> > >> I plan on sending a diff next week to improve the situation via RDTSCP > >> on the machines that have it. Which is basically all modern machines. > >> > >> The plan is to have an auxiliary value returned by RDTSCP which > >> identifies the CPU we got the info from so that we can look-up its > >> associated skew in a table saved at init inside the timekeep structure: > > > > I think that is the wrong approach. Instead we should synchronize the > > TSC counters themselves. There are special MSRs you can write the > > offset into IIRC. That seems to be what FreeBSD does. > > Yes, that is another option. I have not looked to see which are more > popular in terms of hardware. Did the MSRs come with RDTSCP? Before? Or > after? We should choose the most inclusive solution I guess. Or we could > have both...
One thing to keep in mind is that we only use the TSC as a timecounter on CPUs that have the ITSC feature.
