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Re: [Xen-devel] [PATCH v2 12/30] xen/x86: Generate deep dependencies of features

On 15/02/16 15:52, Jan Beulich wrote:
>>>> --- a/xen/tools/gen-cpuid.py
>>>> +++ b/xen/tools/gen-cpuid.py
>>>> @@ -138,6 +138,61 @@ def crunch_numbers(state):
>>>>      state.hvm_shadow = featureset_to_uint32s(state.raw_hvm_shadow, 
>> nr_entries)
>>>>      state.hvm_hap = featureset_to_uint32s(state.raw_hvm_hap, nr_entries)
>>>> +    deps = {
>>>> +        XSAVE:
>>>> +
>>>> +        AVX:
>>>> +        (FMA, FMA4, F16C, AVX2, XOP),
>>> I continue to question whether namely XOP, but perhaps also the
>>> others here except maybe AVX2, really is depending on AVX, and
>>> not just on XSAVE.
>> I am sure we have had this argument before.
> Indeed, hence the "I continue to ...".
>> All VEX encoded SIMD instructions (including XOP which is listed in the
>> same category by AMD) are specified to act on 256bit AVX state, and
>> require AVX enabled in xcr0 to avoid #UD faults.  This includes VEX
>> instructions encoding %xmm registers, which explicitly zero the upper
>> 128bits of the associated %ymm register.
>> This is very clearly a dependency on AVX, even if it isn't written in
>> one clear concise statement in the instruction manuals.
> The question is what AVX actually means: To me it's an instruction set
> extension, not one of machine state. The machine state extension to
> me is tied to XSAVE (or more precisely to XSAVE's YMM state). (But I
> intentionally say "to me", because I can certainly see why this may be
> viewed differently.)

The AVX feature bit is also the indicator that the AVX bit may be set in
XCR0, which links it to machine state and not just instruction sets.

>  Note how you yourself have recourse to XCR0,
> which is very clearly tied to XSAVE and not AVX, above (and note also
> that there's nothing called AVX to be enabled in XCR0, it's YMM that
> you talk about).

The key point is this.  If I choose to enable XSAVE and disable AVX for
a domain, that domain is unable to FMA/FMA4/F16C instructions.  It
therefore shouldn't see the features.


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