Re: [Xen-devel] [PATCH] xen/arm: remove check for generic timer support for arm64

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 06/02/2014 02:03 PM, Ian Campbell wrote:
> On Mon, 2014-06-02 at 12:13 +0100, Julien Grall wrote:
>> On 06/02/2014 12:09 PM, Ian Campbell wrote:
>>> On Mon, 2014-06-02 at 14:07 +0530, vijay.kilari@xxxxxxxxx wrote:
>>>> From: Vijaya Kumar K <Vijaya.Kumar@xxxxxxxxxxxxxxxxxx>
>>>>
>>>> arm64 always supports generic timer. So check is not required
>>>> for arm64. For platforms which supports only aarch64 mode this
>>>> check always passes and panics
>>>
>>> Ah, because the relevant feature flag/register is 32-bit only.
>>>
>>> I'd prefer to see this done in the cpufeature header as:
>>>         #ifdef CONFIG_ARM_32
>>>         #define cpu_has_gentimer  (boot_cpu_feature32(gentimer) == 1)
>>>         #else
>>>         #define cpu_has_gentimer  (1)
>>>         #endif
>>> rather than adding #ifdef to the common code. Likewise for any similar
>>> "always on for aarch64" features.
>>
>> AFAIU, the feature flag exists on ARMv8 platform with aarch32 support.
>> So an ifdef may not be the correct solution here.
> 
> The flag might exist in the AArch32 feature registers (for compat with
> v7) but AIUI the feature is not actually optional on v8.

The manual says the ARM Generic Timer is an optional extension to an
ARMv8 implementation.

> Unless you think it is within the ARMv8 spec for a processor to
> implement generic timers only when running in AArch64 mode and not
> expose them to AArch32 mode? I don't think that is allowed.

I can't find anything in the manual about the Generic Timer is required
when AArch64 is supported.

Anyway, I think it's harmless to check this bit on ARMv8 which support
aarch32.

Regards,

-- 
Julien Grall

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