linux-kernel - Re: [PATCH 08/36] AArch64: Kernel booting and initialisation

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Message-ID: <5009044E.9060908@jonmasters.org>
Date:	Fri, 20 Jul 2012 03:10:06 -0400
From:	Jon Masters <jonathan@...masters.org>
To:	Christopher Covington <cov@...eaurora.org>
CC:	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 08/36] AArch64: Kernel booting and initialisation

On 07/19/2012 01:31 PM, Christopher Covington wrote:
> On 07/18/2012 02:57 AM, Jon Masters wrote:
>> On 07/06/2012 05:05 PM, Catalin Marinas wrote:
>>
>>> +- CPU mode
>>> +  All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
>>> +  IRQ and FIQ).
>>> +  The CPU must be in either EL2 (RECOMMENDED) or non-secure EL1.
> 
> Why not secure EL1?

Because secure world and non-secure world are separated. Although ARMv8
does define EL0 and EL1 in both secure and non-secure worlds, they're
really two different things. General purpose OSes run their kernel in
EL1 (userspace in EL0). We don't ever even see the secure EL1.

>> Even though this stuff is likely to be replaced with the result of some
>> of the other standardization, I'd like it if you'd strongly consider
>> removing the "or non-secure EL1". If you give an inch, someone will take
>> a mile and build a system that enters other than in EL2. Or, something
>> to the effect of "the highest non-secure exception level implemented"
>> would be my preference if you don't want to specify.
> 
> I think it would be best to list the technical limitations, from the
> kernel's perspective, of the unsupported exception levels and the
> advantages of the supported exception levels here. If you want to guide
> system builders towards EL2, I think it'd be more convincing to document
> the relevant technical aspects (perhaps KVM needs facilities only
> available in EL2) than just providing an unexplained requirement.

Unless you enter at EL2 you can never install a hypervisor. That's the
reason for the requirement for generally entering at EL2 when possible.

Jon.

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