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 21:09:55 +0800
From:	cinifr <cinifr@...il.com>
To:	Marc Zyngier <marc.zyngier@....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.

> 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?
Thanks.
BTW: And I fond xen have support to trap accessing   CNTPCT by this
patch on http://lists.xen.org/archives/html/xen-devel/2012-08/msg00452.html.

> All you can do is disable access to the physical timer/counter,
> resulting in an UNDEF in the *guest*.
>
> Additionally, please realise the overhead of trapping is enormous, and
> that we do try very hard to minimise it. Why do you think we went out of
> our way to ensure that host and guest would use different timers, always?

>
> CNTVOFF can only be setup from either HYP or Secure Monitor mode with
> SCR.NS == 1, so if you run your kernel in secure mode, it is always best
> to do it in the bootloader.
I agree it, Bootloader do  it better.

         Fan
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