[PATCH] arm64/cpufeatures: Enforce inline/const properties of cpus_have_const_cap

[Marc Zyngier]

On 19/01/17 14:37, Will Deacon wrote:
> On Wed, Jan 18, 2017 at 11:58:45AM +0000, Marc Zyngier wrote:
>> Despite being flagged "inline", cpus_have_const_cap may end-up being
>> placed out of line if the compiler decides so. This would be unfortunate,
>> as we want to be able to use this function in HYP, where we need to
>> be 100% sure of what is mapped there. __always_inline seems to be a
>> better choice given the constraint.
>>
>> Also, be a lot tougher on non-const or out-of-range capability values
>> (a non-const cap value shouldn't be used here, and the semantic of an
>> OOR value is at best ill defined). In those two case, BUILD_BUG_ON is
>> what you get.
>>
>> Reviewed-by: Suzuki K Poulose <suzuki.poulose at arm.com>
>> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
>> ---
>>  arch/arm64/include/asm/cpufeature.h | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>> index b4989df..4710469 100644
>> --- a/arch/arm64/include/asm/cpufeature.h
>> +++ b/arch/arm64/include/asm/cpufeature.h
>> @@ -105,10 +105,11 @@ static inline bool cpu_have_feature(unsigned int num)
>>  }
>>  
>>  /* System capability check for constant caps */
>> -static inline bool cpus_have_const_cap(int num)
>> +static __always_inline bool cpus_have_const_cap(int num)
>>  {
>> -	if (num >= ARM64_NCAPS)
>> -		return false;
>> +	BUILD_BUG_ON(!__builtin_constant_p(num));
>> +	BUILD_BUG_ON(num >= ARM64_NCAPS);
> 
> This gives different behaviour to cpus_have_const_cap when compared to
> cpus_have_cap, which I really don't like. What is the current behaviour
> if you pass a non-constant num parameter? Does the kernel actually build?

If your toolchain doesn't support jump labels (gcc 4.8 for example), it
will build. But my point here is that if you're using the _const
version, it should to be an actual constant, within the range of
existing capabilities. Otherwise, I don't really understand what the
semantic of _const means here.

> Maybe it's best to spin a separate patch that makes cpus_have_cap and
> cpus_have_const_cap both use __always_inline, then we can debate the merit
> of the BUILD_BUG_ONs separately.

Sure, will do.

Thanks,

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