linux-kernel - Re: [PATCH 5/9] clocksource: tegra: Enable ARM arch

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Message-ID: <50D31365.9000009@arm.com>
Date:	Thu, 20 Dec 2012 13:32:21 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Peter De Schrijver <pdeschrijver@...dia.com>
CC:	Hiroshi Doyu <hdoyu@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"andrew@...n.ch" <andrew@...n.ch>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	"johnstul@...ibm.com" <johnstul@...ibm.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 5/9] clocksource: tegra: Enable ARM arch_timer with TSC

On 20/12/12 12:55, Peter De Schrijver wrote:
> On Thu, Dec 20, 2012 at 01:33:42PM +0100, Marc Zyngier wrote:
>> On 20/12/12 12:22, Peter De Schrijver wrote:
>>>>>>> +
>>>>>>> +	/* CNTFRQ */
>>>>>>> +	asm("mcr p15, 0, %0, c14, c0, 0\n" : : "r" (freq));
>>>>>>> +	asm("mrc p15, 0, %0, c14, c0, 0\n" : "=r" (val));
>>>>>>> +	BUG_ON(val != freq);
>>>>>>
>>>>>> This is scary. CNTFRQ is only writable from secure mode, and will
>>>>>> explode in any other situation.
>>>>>>
>>>>>> Also, writing to CNTFRQ doesn't change the timer frequency! This is just
>>>>>> a way for secure mode to tell the rest of the world the frequency the
>>>>>> timer is ticking at. Unless you've wired the input clock to be able to
>>>>>> change the frequency?
>>>>>
>>>>> ATM, our upstream kernel is expected in secure mode. This situation
>>>>> may be changed later, though....
>>>>
>>>> I appreciate this. But I expect this kernel to be also used on the
>>>> non-secure side if someone tried to run KVM with it. And this would go
>>>> bang right away.
>>>>
>>>
>>> But the guest wouldn't necessarily have this peripheral, or any other Tegra114
>>> peripheral for that matter?
>>
>> The problem is not so much the guest but the host. The host has to be
>> booted in non-secure, so just saying "we do not support non-secure" is
>> not a very convincing argument.
>>
>> Unless of course you've already decided that you don't want to support
>> KVM on this SoC...
>>
> 
> I guess that means we can't support KVM yet. Tegra does not have a secure
> monitor by default. It all depends on what that system integrator does.

VExpress doesn't have a secure monitor either, and yet we run KVM on it
(by switching to non-secure before loading the kernel). Same for Exynos5.

What I'm trying to say is that this code is rather pointless (this
should be done by the firmware/bootloader, not the kernel, or the
information should be provided in DT if CNTFRQ is not set).

This way, no breakage, no dependency on the security level.

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

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