Re: [PATCH 01/10] Support Intel AVX-IFMA

[Jan Beulich <jbeulich@suse.com>]

On 18.10.2022 00:23, H.J. Lu wrote:
> On Sat, Oct 15, 2022 at 11:39 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 14.10.2022 20:10, H.J. Lu wrote:
>>> On Fri, Oct 14, 2022 at 2:52 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>
>>>> On 14.10.2022 11:12, Haochen Jiang wrote:
>>>>> From: wwwhhhyyy <hongyu.wang@intel.com>
>>>>>
>>>>> x86: Support Intel AVX-IFMA
>>>>>
>>>>> Intel AVX IFMA instructions are marked with CpuVEX_PREFIX, which is
>>>>> cleared by default.  Without {vex} pseudo prefix, Intel IFMA instructions
>>>>> are encoded with EVEX prefix.  {vex} pseudo prefix will turn on VEX
>>>>> encoding for Intel IFMA instructions.
>>>>
>>>> I firmly object to the proliferation of this mis-feature. As expressed
>>>> before for AVX-VNNI, as long as the user has disabled AVX512 (or
>>>> respective sub-features thereof), there should be no need to use {vex} in
>>>> the source code. There's also no reason at all to make the disassembler
>>>> print {vex} prefixes - we don't do so for any other insns (apart from
>>>> AVX-VNNI) where an ambiguity exists between their VEX and EVEX encodings
>>>> (when none of the EVEX-specific features is used).
>>>
>>> The {vex} prefix is used with AVX-IFMA instructions so that IFMA instructions
>>> without a prefix, which are generated by compilers or handwritten, will be
>>> always encoded with EVEX.
>>
>> So again: Why is this necessary when a programmer disabled AVX512? I fully
>> agree we need to pick the EVEX encoding by default if available, but I see
>> no reason whatsoever to insist on a {vex} prefix when the EVEX variant is
>> unavailable anyway. As you said back at the time for AVX-VNNI - this was a
>> design decision taken at Intel. Which is fine for a draft implementation.
>> But decisions for an open source project should be taken in the open, and
>> opinions of others should not simply be put off.
>>
> 
> We can discuss how to initialize i.vec_encoding.  But it is orthogonal to
> this patch.

One can view it as orthogonal, yes, but if we change the model then doing
so before more code and testcases need changing is imo preferable.

Jan