RE: [tip:timers/core] clocksource/drivers: Make Hyper-V clocksource ISA agnostic

From: Thomas Gleixner <tglx@linutronix.de>
To: Dexuan Cui <decui@microsoft.com>
Cc: "mark.rutland@arm.com" <mark.rutland@arm.com>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"rkrcmar@redhat.com" <rkrcmar@redhat.com>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	"linux@rasmusvillemoes.dk" <linux@rasmusvillemoes.dk>,
	"linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
	"linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	KY Srinivasan <kys@microsoft.com>,
	"shuah@kernel.org" <shuah@kernel.org>,
	"mingo@kernel.org" <mingo@kernel.org>,
	"sashal@kernel.org" <sashal@kernel.org>,
	"sfr@canb.auug.org.au" <sfr@canb.auug.org.au>,
	"vincenzo.frascino@arm.com" <vincenzo.frascino@arm.com>,
	"linux-tip-commits@vger.kernel.org"
	<linux-tip-commits@vger.kernel.org>,
	"daniel.lezcano@linaro.org" <daniel.le>
Subject: RE: [tip:timers/core] clocksource/drivers: Make Hyper-V clocksource ISA agnostic
Date: Sat, 6 Jul 2019 09:56:32 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1907060944200.3648@nanos.tec.linutronix.de> (raw)
In-Reply-To: <PU1P153MB0169E8E3898897BF3B3BE0F6BFF40@PU1P153MB0169.APCP153.PROD.OUTLOOK.COM>

On Sat, 6 Jul 2019, Dexuan Cui wrote:
> > From: linux-hyperv-owner@vger.kernel.org
> > <linux-hyperv-owner@vger.kernel.org> On Behalf Of tip-bot for Michael Kelley
> > No behavior is changed and no new functionality is added.
> >... 
> > +void hv_stimer_cleanup(unsigned int cpu)
> > +{
> > +	struct clock_event_device *ce;
> > +
> > +	/* Turn off clockevent device */
> > +	if (ms_hyperv.features & HV_MSR_SYNTIMER_AVAILABLE) {
> > +		ce = per_cpu_ptr(hv_clock_event, cpu);
> > +		hv_ce_shutdown(ce);
> > +	}
> > +}
> 
> Actually, there is a behavior change (see https://lkml.org/lkml/2019/7/1/9):
> 	Removed call to clockevents_unbind_device() [Thomas Gleixner]
> and this breaks my ongoing work of Linux guest hibernation on Hyper-V. :-(
> 
> I guess Thomas suggested we remove the call because the clockevent device
> can be automatically cleaned up in tick_cleanup_dead_cpu(), when a CPU is 
> offlined? BTW, I didn't find Thomas's mail suggesting the change.
> 
> In the case of hibernation, this is not true for CPU0, because 
> tick_cleanup_dead_cpu() is not called for CPU0. As a result, when the
> system resumes from hibernation, we add the same clock_event_device
> into the global list clockevent_devices twice in hv_stimer_init(0) ->
> clockevents_config_and_register() for CPU0, causing unexpected
> behaviors...

The way this is normally done is to suspend the clockevent device on
hibernation and resume it when coming back. From a kernel POV resume is
really different than boot.

But that needs probably more surgery. I'll add the unbind() call back for
now.

Thanks,

	tglx