RE: [patch 19/48] clockevents: Provide support for clocksource coupled comparators

[Thomas Gleixner]

On Mon, Mar 23 2026 at 04:24, Michael Kelley wrote:
> From: Michael Kelley Sent: Tuesday, March 3, 2026 10:45 AM
>> From: Thomas Gleixner <tglx@xxxxxxxxxx> Sent: Tuesday, February 24, 2026 8:37 AM
>> >
>> > Some clockevent devices are coupled to the system clocksource by
>> > implementing a less than or equal comparator which compares the programmed
>> > absolute expiry time against the underlying time counter.
>> 
>> I've been playing with this in linux-next, and particularly to set up the Hyper-V
>> TSC page clocksource and Hyper-V timer as coupled. Most Hyper-V guests these days
>> are running on hardware that allows using the TSC directly as the clocksource. But
>> even if the Hyper-V TSC page clocksource isn't used, the timer is still the Hyper-V
>> timer, so the coupling isn't active. However, SEV-SNP and TDX CoCo VMs on Hyper-V
>> must use both the Hyper-V TSC page clocksource and the Hyper-V timer, so they
>> would benefit from coupling. It's a nice idea!

Did not think about that. I try to avoid the virt dungeon as much as it
goes :)

>> > +static inline bool clockevent_set_next_coupled(struct clock_event_device *dev, ktime_t expires)
>> > +{
>> > +	u64 cycles;
>> > +
>> > +	if (unlikely(!(dev->features & CLOCK_EVT_FEAT_CLOCKSOURCE_COUPLED)))
>> > +		return false;
>> > +
>> > +	if (unlikely(!ktime_expiry_to_cycles(dev->cs_id, expires, &cycles)))
>> > +		return false;
>> > +
>> > +	if (IS_ENABLED(CONFIG_GENERIC_CLOCKEVENTS_COUPLED_INLINE))
>> 
>> Since COUPLED_INLINE is always selected for x64, there's no way to add the Hyper-V
>> clockevent that is coupled but not inline. Adding the machinery to allow a second
>> inline clockevent type may not be worth it, but adding a second coupled but not
>> inline clockevent type on x64 should be supported. Thoughts?
>> 
>> After fixing the u64 typo, and temporarily not always selecting COUPLED_INLINE in
>> arch/x86/Kconfig, the coupled Hyper-V TSC page clocksource and timer seem to work
>> correctly, though I'm still doing some testing. I'm also working on counting the number
>> of time reads to confirm the expected benefit.
>
> Gentle ping.  Any thoughts on this?  (And on Peter Zijlstra's "deliciously insane"
> follow-up?)

Sure, we should be able to support that and I think Peter's suggestion
is pretty clever. Did you get it working?

Thanks,

        tglx