Re: [Xen-devel] [PATCH v4] x86/cpuid: AVX-512 Feature Detection

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/07/16 03:42, Kang, Luwei wrote:
>
>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Tuesday, July 5, 2016 3:03 PM
>> To: Kang, Luwei <luwei.kang@xxxxxxxxx>
>> Cc: andrew.cooper3@xxxxxxxxxx; chao.p.peng@xxxxxxxxxxxxxxx; xen-
>> devel@xxxxxxxxxxxxx
>> Subject: RE: [PATCH v4] x86/cpuid: AVX-512 Feature Detection
>>
>>>>> On 05.07.16 at 04:31, <luwei.kang@xxxxxxxxx> wrote:
>> First of all - please don't top post.
>>
>>> What about remove the dependency between AVX2 and AVX512F ( AVX2:
>> [AVX512F], ) ?
>>
>> Yes, that's what I think we want, but we need Andrew's agreement here.
>>
> Hi Andrew,  what is your opinion ?

You are in a better position to answer than me.

For a specific instruction which may be VEX and EVEX encoded, is the
circuitry for a specific instruction shared, or discrete?  Is there a
plausible future where you might support only the EVEX variant of an
instruction, and not the VEX variant?

These dependences are about what may be reasonably assumed about the way
the environment is structured.  It doesn't look reasonable to advertise
an AVX512 environment to guests while at the same time stating that AVX2
is not present.  If this is correct, then the dependency should stay. 
If Intel plausibly things it might release hardware with !AVX2 but
AVX512, then the dependency should be dropped.

~Andrew

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