Re: [Xen-devel] [PATCH 02/10] x86/gen-cpuid: Clarify the intended meaning of AVX wrt feature dependencies

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/02/17 16:47, Jan Beulich wrote:
>>>> On 21.02.17 at 17:40, <JBeulich@xxxxxxxx> wrote:
>>>>> On 20.02.17 at 12:00, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> --- a/xen/tools/gen-cpuid.py
>>> +++ b/xen/tools/gen-cpuid.py
>>> @@ -225,9 +225,13 @@ def crunch_numbers(state):
>>>          XSAVE: [XSAVEOPT, XSAVEC, XGETBV1, XSAVES,
>>>                  AVX, MPX, PKU, LWP],
>>>  
>>> -        # AVX is taken to mean hardware support for VEX encoded 
>>> instructions,
>>> -        # 256bit registers, and the instructions themselves.  Each of these
>>> -        # subsequent instruction groups may only be VEX encoded.
>>> +        # AVX is taken to mean hardware support for 256bit registers 
>>> (which in
>>> +        # practice depends on the VEX prefix to encode), and the 
>>> instructions
>>> +        # themselves.
>>> +        #
>>> +        # AVX is not taken to mean support for the VEX prefix itself.
>>> +        # VEX-encoded GPR instructions, such as those from the BMI{1,2} 
>>> sets
>>> +        # function fine in the absence of any enabled xstate.
>>>          AVX: [FMA, FMA4, F16C, AVX2, XOP],
>> Even if there aren't any EVEX-encoded non-AVX512 instructions so
>> far, I'd prefer the AVX512F entry to get adjusted along the same
>> lines. With that
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> Actually, one more thing: XOP really has dual meaning (encoding
> and certain SIMD instructions). Perhaps it would be good to clarify
> this here too.

We don't currently express any dependencies based on XOP, so there is no
text about it.

As AMD have dropped XOP encoding in their Zen line, I don't expect this
will change going forwards.

~Andrew

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