Re: [PATCH v2 08/38] clocksource: hyper-v: Register sched_clock save/restore iff it's necessary

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 04, 2025, Michael Kelley wrote:
> From: Sean Christopherson <seanjc@google.com> Sent: Wednesday, February 26, 2025 6:18 PM
> > 
> > Register the Hyper-V timer callbacks or saving/restoring its PV sched_clock
> 
> s/or/for/
> 
> > if and only if the timer is actually being used for sched_clock.
> > Currently, Hyper-V overrides the save/restore hooks if the reference TSC
> > available, whereas the Hyper-V timer code only overrides sched_clock if
> > the reference TSC is available *and* it's not invariant.  The flaw is
> > effectively papered over by invoking the "old" save/restore callbacks as
> > part of save/restore, but that's unnecessary and fragile.
> 
> The Hyper-V specific terminology here isn't quite right.  There is a
> PV "Hyper-V timer", but it is loaded by the guest OS with a specific value
> and generates an interrupt when that value is reached.  In Linux, it is used
> for clockevents, but it's not a clocksource and is not used for sched_clock.
> The correct Hyper-V term is "Hyper-V reference counter" (or "refcounter"
> for short).  The refcounter behaves like the TSC -- it's a monotonically
> increasing value that is read-only, and can serve as the sched_clock.
> 
> And yes, both the Hyper-V timer and Hyper-V refcounter code is in a
> source file with a name containing "timer" but not "refcounter". But
> that seems to be the pattern for many of the drivers in
> drivers/clocksource. :-)

Heh, wading through misleading naming is basically a right of passage in the kernel.

Thanks for the reviews and testing!  I'll fixup all the changelogs.