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

["Kang, Luwei" <luwei.kang@xxxxxxxxx>]

> >> 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.

Regarding the dependency between AVX2 and AVX512F, I have ask some hardware 
architecture engineer.

AVX512 is a superset of AVX2, the most important item being the state. If the 
state for AVX2 isn't enabled (in XCR0), then AVX512 also can't function.

So if we want to use AVX512, AVX2 must be supported and enabled. The dependence 
between AVX512F and AVX2 may need be reserved.

> 
> ~Andrew

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