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

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Date:	Fri, 13 Sep 2013 14:40:29 +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>,
	"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 13/09/13 14:09, cinifr wrote:
>> I urge you to read the ARM ARM, and specifically the section dedicated
>> to trapping access to CP15 operations. If you do, you'll quickly notice
>> that you *cannot* trap accesses to the timer subsystem.
>>
> I read it again. The ARMv7 manual said "Is accessible from Non-secure
> PL1 modes only when CNTHCTL.PL1PCTEN is set to 1. When
> CNTHCTL.PL1PCTEN is set to 0, any attempt to access CNTPCT from a
> Non-secure PL1 mode ***generates a Hyp Trap exception***, see Hyp Trap
> exception on page B1-1206" in B8.1.2. but I dont find  a  special hyp
> trap control  for accessing CNTPCT in manual. As you said HSTR cannot
> trap accessing of CP15 c14. What happer when OS access CNTPCT from PL1
> NS=1 mode with CNTHCTL.PL1PCTEN=0 ???  AmI  wrong for understanding
> the manual?

That's interesting, as I never noticed this particular line in the ARM
ARM. I'll investigate this, thanks for bringing it up.

This doesn't change the fact that using the physical timer/counter in a
VM is (or can be) horribly expensive, and should be avoided at all cost.

Thanks,

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

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