Re: [Xen-devel] [PATCH] x86/PV: hide features dependent on XSAVE when booted with "no-xsave"

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.11.15 at 14:36, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 30/11/15 11:30, Jan Beulich wrote:
>> It's not well defined whether YMM register presence
>> correlates to AVX, or is simply flagged by the respective XSTATE
>> CPUID bit (or a mixture of both).
> 
> It is indeed not well defined, which is what makes this area of
> functionality so hard to level safely.
> 
>> The minimal (and imo more natural) dependency is just the XSTATE bit.
> 
> But it is wrong.
> 
> Any VEX encoded SIMD operation unconditionally works on YMM state.  In
> the case that XMM registers are encoded with a VEX prefix, the upper 128
> bits of the YMM register are zeroed (SDM Vol 2, 2.3.10).  This is
> contrary to legacy SSE instructions which preserve the upper 128 bits.
> 
> Therefore, FMA, FMA4 and XOP do have a strict dependency on AVX.

No, if you really want to express it that way, you'll need feature
flags derived from the XSTATE bits.

Jan


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