linux-kernel - Re: [PATCH v2 08/38] clocksource: hyper-v: Register sched

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Message-ID: <Z8c641D3AuWNXGVB@google.com>
Date: Tue, 4 Mar 2025 09:39:47 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Michael Kelley <mhklinux@...look.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Juergen Gross <jgross@...e.com>, "K. Y. Srinivasan" <kys@...rosoft.com>, 
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>, Ajay Kaher <ajay.kaher@...adcom.com>, 
	Jan Kiszka <jan.kiszka@...mens.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	John Stultz <jstultz@...gle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>, 
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>, 
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>, Tom Lendacky <thomas.lendacky@....com>, 
	Nikunj A Dadhania <nikunj@....com>
Subject: Re: [PATCH v2 08/38] clocksource: hyper-v: Register sched_clock
 save/restore iff it's necessary

On Tue, Mar 04, 2025, Michael Kelley wrote:
> From: Sean Christopherson <seanjc@...gle.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.