Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support

[Peter Zijlstra]

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 weâre in a KVM guest, then the clock will jump around on the
> same *vCPU* when the vCPU migrates.

Urgh yes..

> But I donât see how kvmclock helps here, since I donât think itâs 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.