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

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Message-ID: <BYAPR21MB122115920E78B7897FDC7BE9D7180@BYAPR21MB1221.namprd21.prod.outlook.com>
Date:   Thu, 30 May 2019 12:39:33 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     vkuznets <vkuznets@...hat.com>
CC:     "catalin.marinas@....com" <catalin.marinas@....com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "marc.zyngier@....com" <marc.zyngier@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>
Subject: RE: [PATCH v3 2/2] Drivers: hv: Move Hyper-V clocksource code to new
 clocksource driver

From: Vitaly Kuznetsov <vkuznets@...hat.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.

Michael