Hi Thomas, On 26/06/2019 07:38, Thomas Gleixner wrote: > On Tue, 25 Jun 2019, Thomas Gleixner wrote: >> On Tue, 25 Jun 2019, Vincenzo Frascino wrote: >>> do_hres() in the vDSO generic library masks the hw counter value >>> immediately after reading it. >>> >>> Postpone the mask application after checking if the syscall fallback is >>> enabled, in order to be able to detect a possible fallback for the >>> architectures that have masks smaller than ULLONG_MAX. >> >> Right. This only worked on x86 because the mask is there ULLONG_MAX for all >> VDSO capable clocksources, i.e. that ever worked just by chance. > > But it's actually worse than that: > >>> + cycles &= vd->mask; >>> if (cycles > last) >>> ns += (cycles - last) * vd->mult; >>> ns >>= vd->shift; > > This is broken for any clocksource which can legitimately wrap around. The > core timekeeping does the right thing: > > (cycles - last) & mask > > That makes sure that a wraparound is correctly handled. With the above the > wrap around would be ignored due to > > if (cycles > last) >
You are right. Thanks for spotting it. ... -- Regards, Vincenzo
