Re: [PATCH 8/8] Support APX JMPABS

[Jan Beulich <jbeulich@suse.com>]

On 27.11.2023 09:46, Hu, Lin1 wrote:
>>>>>>>> Also a spec question as we're talking of what is or is not valid (i.e.
>>>>>>>> causing #UD) here: Why would XCR0.APX=0 need to cause #UD?
>>>>>>>> There's no use of eGPR-s here.
>>>>>>>>
>>>>>>>
>>>>>>> Sorry, what is XCR0.APX?
>>>>>>
>>>>>> Bit 19 of the XCR0 register. It is mentioned in exactly this way in
>>>>>> the APX- LEGACY-JMPABS exception class description.
>>>>>>
>>>>>
>>>>> I think XCR0.APX is a state bit to control if support APX
>>>>> instruction set not if
>>>> support eGPR-s.
>>>>
>>>> No, XCR0 very certainly is a set of controls affecting register use.
>>>> If JMPABS is also controlled by it, then I'd view this as an erratum
>>>> if it appeared like that in silicon; that erratum may well be a
>>>> design one then, or one "justified" by simplifying the implementation
>>>> in some way, but it would still be wrong from a conceptual pov.
>>>>
>>>
>>> In APX.pdf (https://cdrdv2.intel.com/v1/dl/getContent/784266) section
>>> 3.1.4.2, XCR0 govern APX State and prefixes. And In sdm.pdf
>> (https://cdrdv2.intel.com/v1/dl/getContent/671200), section 13.3 page 323,
>> "Software can excute Intel AVX-512 instructions only if CR4.OSXSAVE = 1 and
>> XCR0[7:5] = 111b." Their focus is on XCR0[7:5], not on what corresponding
>> registers are used, For example, page 567 Table 2-37, alougth bit 6 is used for
>> the upper 256 bits of the register ZMM0-ZMM15, 256 encoding instructions still
>> require to set the bit.
>>>
>>> I admit that in the earliest introductions of XCR0[7:5], it looks like it's for
>> register state use only, but from actual use later on, they're more of an
>> instruction state, but from the introductions of the XCR0.APX bit, that's not a
>> problem.
>>
>> Since you take AVX512 for analogy: Can you please point me at an insn which
>> doesn't use any AVX512 register covered by said three XCR0 bits? Talking of insn
>> use and talking of register use simply is the same there. Hence the analogy
>> cannot be used when discussing JMPABS. Furthermore the specific wording in
>> SDM, ISE, or APX doc also cannot be blindly trusted. What matters is how silicon
>> is going to behave, and for JMPABS my impression is that if a dependency on
>> XCR0 was existing there, it would have been introduced artificially, i.e. without
>> real need. _That's_ what I'm putting under question.
>>
> 
> If I use "{evex} vaddpd ymm0, ymm1, ymm2". If it's for register considerations, I think bit 5, 6, 7 can all be zero. Because the insn doesn't use k0-k7 and the upper 256 bits of the registers ZMM0-ZMM15. 

How that? It clears the upper 256 bits of the destination register.

Jan