On Thu, 4 Sep 2014, Paolo Bonzini wrote: > Commit cbcf2dd3b3d4 (x86: kvm: Make kvm_get_time_and_clockread() nanoseconds > based, 2014-07-16) used the wrong formula for boot_ns, thus breaking kvmclock on > hosts that have a reliable TSC. > > To find the right formula, let's first backport the switch to nanoseconds > to 3.16-era timekeeping logic. The full patch (which works) is at > https://lkml.org/lkml/2014/9/4/462. The key line here is > > boot_ns = timespec_to_ns(&tk->total_sleep_time) > + timespec_to_ns(&tk->wall_to_monotonic) > + tk->xtime_sec * (u64)NSEC_PER_SEC; > > Because the above patch works, the conclusion is that the above formula > is not the same as commit cbcf2dd3b3d4's > > boot_ns = ktime_to_ns(ktime_add(tk->tkr.base_mono, tk->offs_boot)); > > As to what is the right one, commit 02cba1598a2a (timekeeping: Simplify getboottime(), > 2014-07-16) provides a hint: > > offs_real = -wall-to_monotonic > offs_boot = total_sleep_time > > offs_real - offs_boot = -wall_to_monotonic - total_sleep_time > > that is > > offs_boot - offs_real = wall_to_monotonic + total_sleep_time > > which is what this patch uses, adding xtime_sec separately. The "boot_ns" > moniker is not too clear, so rename boot_ns to nsec_base and the existing > nsec_base to snsec_base. > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: John Stultz <john.stultz@xxxxxxxxxx> > Reported-by: Chris J Arges <chris.j.arges@xxxxxxxxxxxxx> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > --- > Thomas/John, the problem with the above explanation is that > tk_update_ktime_data has "base_mono = xtime_sec + wtm", and from > there "base_mono + offs_boot = xtime_sec + wtm + total_sleep_time". > Except that doesn't work, so something must be wrong in > tk_update_ktime_data's comment. Right. I'm staring into it and we need to fix the core code issue and not the usage site. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
