linux-kernel - Re: [PATCH 3/4] Add physical count arch timer support for clocksource in ARMv7.

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Message-ID: <5231EE43.5090900@arm.com>
Date:	Thu, 12 Sep 2013 17:39:31 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	cinifr <cinifr@...il.com>
CC:	Mark Rutland <Mark.Rutland@....com>,
	"coosty@....com" <coosty@....com>,
	"maxime.ripard@...e-electrons.com" <maxime.ripard@...e-electrons.com>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"pawel.moll@....co" <pawel.moll@....co>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"linux-sunxi@...glegroups.com" <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH 3/4] Add physical count arch timer support for clocksource
 in ARMv7.

On 12/09/13 17:07, cinifr wrote:
>> This cannot be a compile-time option as above in a multiplatform build.
>> Other paltforms (e.g. KVM guests) *must* use the virtual counters to get
>> any semblance of a consistent view of time.
> Yes I accept compile-time option is not perfect  in my pre email,
> But,Why Ohter paltforms *must* use the virtual counters? I think KVM
> should not limit how to use arch timer in its guest OS. Of cause, if
> KVM guest use vct can be more efficiency then that use pct. but KVM
> should and must support guest OS to access pct.

The virtual counter is there for a good reason: it allows a virtual
machine to:
- see its time starting at zero
- be migrated to another host without seeing time shifting one way or
another.

So using the physical counter in a VM is a recipe for disaster if you're
doing any kind of time tracking. The counter being used for
sched_clock(), we cannot afford to see it being shifted one way or another.

If you have issues with the use of the virtual counter, I suggest you
fix your firmware to have a consistent CNTVOFF across CPUs. And/or even
better, boot your kernel in HYP mode, as it will take care of setting
CNTVOFF to zero.

Cheers,

	M.
-- 
Jazz is not dead. It just smells funny...

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