# Re: \[PATCH 0/2\] clocksource/Hyper-V: Add Hyper-V specific sched clock function

- From: Vitaly Kuznetsov \<vkuznets@redhat.com\>
- Date: 2019-07-29 10:59:32 
- List: linux-hyperv
- Page: https://ratatoskr.run/linux-hyperv/2019/07/5669808
- Thread: https://ratatoskr.run/linux-arch/2019/07/5669607/t.md

```text
lantianyu1986@gmail.com writes:

> From: Tianyu Lan <redacted>
>
> 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().

-- 
Vitaly
```