[PATCH] powerpc: Support auxilliary vector components for cache geometries

[Paul Clarke]

On 03/31/2017 11:52 AM, Florian Weimer wrote:
> * Paul Clarke:
>
>> On 03/30/2017 03:14 PM, Florian Weimer wrote:
>>> * Paul Clarke:
>>>>>> +  val = getauxval (AT_L1I_CACHEGEOMETRY);
>>>>>> +  if (val)
>>>>>> +    printf("AT_L1I_CACHEGEOMETRY: associativity %ld; line size %ld\n",
>>>>>> +	   (val & 0xffff0000) >> 16, val & 0x0000ffff);
>>>>>> +  else
>>>>>> +    rc = EXIT_UNSUPPORTED;
>>>>>
>>>>> honestly, what is the value of this test ?  you basically just
>>>>> printf the values everywhere, or you exit unsupported.  there is
>>>>> no actual "test" here that i can see as you don't validate the
>>>>> results anywhere.
>>>>
>>>> I debated this with colleagues before sending (and perhaps should've
>>>> deferred to their experience).  I was reluctant to add new code
>>>> without exercising it, at least a successful compile and run.  However
>>>> it's difficult to determine a true "failure" case without knowing too
>>>> much about the kernel.  I also like that it provides an example of
>>>> use.  If those reasons are not sufficient, I can also remove it from
>>>> the patch.
>>>
>>> Can you at least add consistency checks which check that the values
>>> use the right endianess?  I think that part could be useful.
>>
>> That seems error-prone.  Given "all possibilities", the values
>> returned in the lowest-order and next-to-lowest-order 16 bits of a
>> long return value from getauxval() are basically arbitrary.  I'm not
>> sure how one could determine correct endianness of an arbitrary value.
>> What values are guaranteed correct or incorrect for associativity and
>> cache line size?
>
> I think for 64-bit at least, a byte-swapped return value might be
> larger than UINT_MAX.  Not sure if it is worth detecting this.

That could catch the case where the entire 64-bit value is not native endianness, but not if just the 16-bit fields are non-native.

Florian, I share your concerns about whether a sufficiently valuable test case can be created with reasonable effort.  Any objection to just skipping a test case here?

PC