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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 30/11/15 15:22, Jan Beulich wrote:
>>>> On 30.11.15 at 14:36, <andrew.cooper3@citrix.com> 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.

What? That is absurd.

They depend on AVX.  I am not introducing a synthetic feature when the
real feature bits are correct.

~Andrew