Subject: [PATCH 03/18] arm64: boot protocol documentation update for GICv3

From: [email-masked] (Marc Zyngier)
Date: Wed, 26 Feb 2014 15:59:31 +0000

On 26/02/14 15:31, Will Deacon wrote:
> On Wed, Feb 26, 2014 at 02:37:00PM +0000, Marc Zyngier wrote:
>> On 25/02/14 18:06, Will Deacon wrote:
>>> On Wed, Feb 05, 2014 at 01:30:35PM +0000, Marc Zyngier wrote:
>>>> Linux has some requirements that must be satisfied in order to boot
>>>> on a system built with a GICv3.
>>>>
>>>> Signed-off-by: Marc Zyngier <[email-masked]>
>>>> ---
>>>>  Documentation/arm64/booting.txt | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
>>>> index a9691cc..4a02ebd 100644
>>>> --- a/Documentation/arm64/booting.txt
>>>> +++ b/Documentation/arm64/booting.txt
>>>> @@ -131,6 +131,13 @@ Before jumping into the kernel, the following conditions must be met:
>>>>    the kernel image will be entered must be initialised by software at a
>>>>    higher exception level to prevent execution in an UNKNOWN state.
>>>>  
>>>> +  For systems with a GICv3 interrupt controller, it is expected that:
>>>> +  - ID_AA64PFR0_EL1.GIC (bits [27:24]) must have the value 0b0001
>>>
>>> Since ID_AA64PFR0_EL1 is read-only at all exception levels, I don't see the
>>> value of this statement.
>>
>> Think virtualization. A hypervisor can control reads of ID_AA64PFR0_EL1
>> by setting HCR_EL2.TID3, and report whatever it wants.
> 
> Sure, but it seems unreasonable to me that we require a hypervisor to tell a
> guest about GICv3 if the system happens to have one. What if it wants to
> emulate a GICv2? In other words, requiring this in booting.txt seems
> superflous.

The hypervisor can perfectly mask out the GICv3 feature if it wants to.
The point I'm trying to make here is that if a guest is expected to be
able to use the GICv3 system registers, then ID_AA64PFR0_EL1 should
report so.

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