On Mon, Oct 08, 2018 at 10:38:22AM -0700, Andy Lutomirski wrote: > On Mon, Oct 8, 2018 at 8:27 AM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > > > On Sat, Oct 06, 2018 at 03:28:05PM -0700, Andy Lutomirski wrote: > > > On Sat, Oct 6, 2018 at 1:29 PM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > > > > > > > On Thu, Oct 04, 2018 at 03:15:32PM -0700, Andy Lutomirski wrote: > > > > > For better or for worse, I'm trying to understand this code. So far, > > > > > I've come up with this patch: > > > > > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso-tglx&id=14fd71e12b1c4492a06f368f75041f263e6862bf > > > > > > > > > > Is it correct, or am I missing some subtlety? > > > > > > > > The master clock, when initialized, has a pair > > > > > > > > masterclockvalues=(TSC value, time-of-day data). > > > > > > > > When updating the guest clock, we only update relative to (TSC value) > > > > that was read on masterclock initialization. > > > > > > I don't see the problem. The masterclock data is updated here: > > > > > > host_tsc_clocksource = kvm_get_time_and_clockread( > > > &ka->master_kernel_ns, > > > &ka->master_cycle_now); > > > > > > kvm_get_time_and_clockread() gets those values from > > > do_monotonic_boot(), which, barring bugs, should cause > > > get_kvmclock_ns() to return exactly the same thing as > > > ktime_get_boot_ns() + ka->kvmclock_offset, albeit in a rather > > > roundabout manner. > > > > > > So what am I missing? Is there actually something wrong with my patch? > > > > For the bug mentioned in the comment not to happen, you must only read > > TSC and add it as offset to (TSC value, time-of-day data). > > > > Its more than "a roundabout manner". > > > > Read the comment again. > > > > I read the comment three more times and even dug through the git > history. It seems like what you're saying is that, under certain > conditions (which arguably would be bugs in the core Linux timing > code), I don't see that as a bug. Its just a side effect of reading two different clocks (one is CLOCK_MONOTONIC and the other is TSC), and using those two clocks to as a "base + offset". As the comment explains, if you do that, can't guarantee monotonicity. > actually calling ktime_get_boot_ns() could be non-monotonic > with respect to the kvmclock timing. But get_kvmclock_ns() isn't used > for VM timing as such -- it's used for the IOCTL interfaces for > updating the time offset. So can you explain how my patch is > incorrect? ktime_get_boot_ns() has frequency correction applied, while reading masterclock + TSC offset does not. So the clock reads differ. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
