From: Vitaly Kuznetsov <vkuznets@redhat•com>
To: Michael Kelley <mikelley@microsoft•com>
Cc: "mark.rutland@arm•com" <mark.rutland@arm•com>,
"linux-hyperv@vger•kernel.org" <linux-hyperv@vger•kernel.org>,
"marc.zyngier@arm•com" <marc.zyngier@arm•com>,
"catalin.marinas@arm•com" <catalin.marinas@arm•com>,
"jasowang@redhat•com" <jasowang@redhat•com>,
"will.deacon@arm•com" <will.deacon@arm•com>,
"linux-kernel@vger•kernel.org" <linux-kernel@vger•kernel.org>,
"marcelo.cerri@canonical•com" <marcelo.cerri@canonical•com>,
"olaf@aepfle•de" <olaf@aepfle•de>,
"gregkh@linuxfoundation•org" <gregkh@linuxfoundation•org>,
"apw@canonical•com" <apw@canonical•com>,
Sunil Muthuswamy <sunilmut@microsoft•com>,
KY Srinivasan <kys@microsoft•com>,
"linux-arm-kernel@lists•infradead.org"
<linux-arm-kernel@lists•infradead.org>
Subject: RE: [PATCH v3 2/2] Drivers: hv: Move Hyper-V clocksource code to new clocksource driver
Date: Thu, 30 May 2019 15:51:05 +0200 [thread overview]
Message-ID: <87imtskq46.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <BYAPR21MB122115920E78B7897FDC7BE9D7180@BYAPR21MB1221.namprd21.prod.outlook.com>
Michael Kelley <mikelley@microsoft•com> writes:
> From: Vitaly Kuznetsov <vkuznets@redhat•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.
Ok, so it is just a fall-back and not going to be actively used. Thanks!
--
Vitaly
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next prev parent reply other threads:[~2019-05-30 13:51 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-27 14:59 [PATCH v3 0/2] Drivers: hv: Move Hyper-V clock/timer code to separate clocksource driver Michael Kelley
2019-05-27 14:59 ` [PATCH v3 1/2] Drivers: hv: Create Hyper-V clocksource driver from existing clockevents code Michael Kelley
2019-05-30 9:37 ` Vitaly Kuznetsov
2019-05-27 14:59 ` [PATCH v3 2/2] Drivers: hv: Move Hyper-V clocksource code to new clocksource driver Michael Kelley
2019-05-30 9:48 ` Vitaly Kuznetsov
2019-05-30 12:39 ` Michael Kelley
2019-05-30 13:51 ` Vitaly Kuznetsov [this message]
2019-06-06 16:37 ` [PATCH v3 0/2] Drivers: hv: Move Hyper-V clock/timer code to separate " Sasha Levin
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