[RFC Linux patch] powerpc: add documentation for HWCAPs

[Paul E Murphy]

On 5/20/22 7:11 PM, Nicholas Piggin wrote:
> Excerpts from Paul E Murphy's message of May 21, 2022 12:21 am:
>>
>>
>> On 5/20/22 12:15 AM, Nicholas Piggin via Gcc wrote:
>>> +PPC_FEATURE2_TAR
>>> +    VSX facility is available.
>>
>> Was manipulating the tar spr was once a privileged instruction, is this
>> a hint userspace can use the related instructions?
> 
> It can be disabled with facility control, and I guess there was
> some consideration for how it might be used, e.g., "system software"
> could use it for its own purpose then clear the bit for the application.
> 
> In practice I don't really know what makes use of this or whether
> anything sanely can, it's marked reserved in the ABI. Would be
> interesting to know whether there is much benefit to use it in the
> compiler. The kernel could actually use it for something nifty if we
> were able to prevent userspace from accessing it entirely...

It might be useful as a scratch register for indirect branches in some 
odd cases, such as golang's preemptive userspace threading.  Though, it 
seems more trouble than its worth for a very limited benefit.

> 
>>> +
>>> +PPC_FEATURE2_HAS_IEEE128
>>> +    IEEE 128 is available? What instructions/data?
>>
>> Maybe something like "IEEE 128 binary floating point instructions are
>> supported.  Individual instruction availability is dependent on the
>> reported architecture version."?
> 
> Right, I just didn't know what architectural class of instructions
> those are. Is it just VSX in general or are there some specific
> things we can name?

I think ISA 3.1 buckets this into an OpenPOWER Linux Optional Feature 
for "Quad-precision floating-point (QFP)".  I guess ISA 3.0 predates 
those categorizations.


>>> +PPC_FEATURE2_MMA
>>> +    MMA facility is available.
>>
>> Maybe another note that specific instruction availability may depend on
>> the reported architecture version?
Yep. I wonder if it would help to note how these align (or don't) with 
the various OpenPOWER features.