RE: [PATCH v3 2/2] Drivers: hv: Move Hyper-V clocksource code to new clocksource driver

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Michael Kelley <mikelley@microsoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Thursday, May 30, 2019 2:48 AM
>> > +		/*
>> > +		 * sched_clock_register is needed on ARM64 but
>> > +		 * is a no-op on x86
>> > +		 */
>> > +		sched_clock_register(read_hv_sched_clock_msr,
>> > +						64, HV_CLOCK_HZ);
>> 
>> I'm not sure about ARM, but MSR-based clocksource would be a really bad
>> choice for sched clock on x86, this will slow things down
>> significantly. Luckily, as you're validly stating above,
>> sched_clock_register() is a no-op on x86 as we don't define
>> CONFIG_GENERIC_SCHED_CLOCK.
>> 
>> Can we actually *not* do sched_clock_register() in case
>> TSC page is unavailable (and revert to counting jiffies or whatever)?
>> 
>
> We can't skip the sched_clock_register() on ARM64 because it
> does define CONFIG_GENERIC_SCHED_CLOCK.  However, Hyper-V
> should always provide REFERENCE_TSC_AVAILALBE on ARM64,
> so we should never end up in the MSR-based code on ARM64.
> Arguably that means the call to sched_clock_register() could be
> removed since it's a no-op on x86.  But I'd like to keep it for symmetry
> and in case there's a testing/debugging situation on ARM64 where
> we want to clear REFERENCE_TSC_AVAILABLE and go down the
> MSR-based code path.

Ok, so it is just a fall-back and not going to be actively used. Thanks!

-- 
Vitaly