Hi Vitaly & Peter:
Thanks for your review.
On Mon, Jul 29, 2019 at 8:13 PM Vitaly Kuznetsov <vkuzn...@redhat.com> wrote:
>
> Peter Zijlstra <pet...@infradead.org> writes:
>
> > On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote:
> >> lantianyu1...@gmail.com writes:
> >>
> >> > From: Tianyu Lan <tianyu....@microsoft.com>
> >> >
> >> > Hyper-V guests use the default native_sched_clock() in
> >> > pv_ops.time.sched_clock
> >> > on x86. But native_sched_clock() directly uses the raw TSC value, which
> >> > can be discontinuous in a Hyper-V VM. Add the generic
> >> > hv_setup_sched_clock()
> >> > to set the sched clock function appropriately. On x86, this sets
> >> > pv_ops.time.sched_clock to read the Hyper-V reference TSC value that is
> >> > scaled and adjusted to be continuous.
> >>
> >> Hypervisor can, in theory, disable TSC page and then we're forced to use
> >> MSR-based clocksource but using it as sched_clock() can be very slow,
> >> I'm afraid.
> >>
> >> On the other hand, what we have now is probably worse: TSC can,
> >> actually, jump backwards (e.g. on migration) and we're breaking the
> >> requirements for sched_clock().
> >
> > That (obviously) also breaks the requirements for using TSC as
> > clocksource.
> >
> > IOW, it breaks the entire purpose of having TSC in the first place.
>
> Currently, we mark raw TSC as unstable when running on Hyper-V (see
> 88c9281a9fba6), 'TSC page' (which is TSC * scale + offset) is being used
> instead. The problem is that 'TSC page' can be disabled by the
> hypervisor and in that case the only remaining clocksource is MSR-based
> (slow).
>
Yes, that will be slow if Hyper-V doesn't expose hv tsc page and
kernel uses MSR based
clocksource. Each MSR read will trigger one VM-EXIT. This also happens on other
hypervisors (e,g, KVM doesn't expose KVM clock). Hypervisor should
take this into
account and determine which clocksource should be exposed or not.
--
Best regards
Tianyu Lan
