On Thu, Oct 04, 2018 at 07:00:45AM -0700, Andy Lutomirski wrote: > > On Oct 4, 2018, at 1:11 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > >> On Thu, Oct 04, 2018 at 09:54:45AM +0200, Vitaly Kuznetsov wrote: > >> I was hoping to hear this from you :-) If I am to suggest how we can > >> move forward I'd propose: > >> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling > >> is supported). > >> - Check if non-masterclock mode is still needed. E.g. HyperV's TSC page > >> clocksource is a single page for the whole VM, not a per-cpu thing. Can > >> we think that all the buggy hardware is already gone? > > > > No, and it is not the hardware you have to worry about (mostly), it is > > the frigging PoS firmware people put on it. > > > > Ever since Nehalem TSC is stable (unless you get to >4 socket systems, > > after which it still can be, but bets are off). But even relatively > > recent systems fail the TSC sync test because firmware messes it up by > > writing to either MSR_TSC or MSR_TSC_ADJUST. > > > > But the thing is, if the TSC is not synced, you cannot use it for > > timekeeping, full stop. So having a single page is fine, it either > > contains a mult/shift that is valid, or it indicates TSC is messed up > > and you fall back to something else. > > > > There is no inbetween there. > > > > For sched_clock we can still use the global page, because the rate will > > still be the same for each cpu, it's just offset between CPUs and the > > code compensates for that. > > But if we’re in a KVM guest, then the clock will jump around on the > same *vCPU* when the vCPU migrates. Urgh yes.. > But I don’t see how kvmclock helps here, since I don’t think it’s used > for sched_clock. I get hits on kvm_sched_clock, but haven't looked further. Anyway, Most of the argument still holds, either TSC is synced or it is not and it _really_ should not be used. Not much middle ground there. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
