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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/11/15 10:01, Jan Beulich wrote:
>>>> On 27.11.15 at 16:05, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 27/11/15 11:05, Jan Beulich wrote:
>>> ... or when the guest has the XSAVE feature hidden by CPUID policy.
>>> Not doing so is at best confusing to guests.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> These changes here are an improvement (so I don't object to taking them
>> ahead of my fullblown levelling series), but they are incomplete.
>>
>> xsaveopt, xsavec, xsetbv1, xsaves, avx and mpx depend on xsave.
>> fma, fma4, f16c, avx2 and xop depend on avx.
> I think the dependencies here are a little fuzzy, and hence I'd
> prefer us to not enforce more strict rules than are truly necessary:
>
> FMA: Neither Intel's SDM nor AMD's PM state any dependency on AVX.
>
> FMA4, XOP: AMD's PM doesn't state any dependency on AVX.
>
> AVX2: While Intel's SDM doesn't say so here either, I agree in this case.
>
> I.e. my view is that FMA{,4} and XOP are all pretty much
> independent of AVX, and they e.g. imply by themselves presence of
> YMM registers.

The AVX feature means several things, and in this case support for VEX
encoded instructions.

Per SDM Vol 2, Table 2-18, any VEX encoded instruction will
unconditionally #UD fault if XCR0[2:1] != '11b' or CR0.OSXSAVE = 0.

FMA, FMA4 and XOP may only be VEX encoded, so do explicitly depend on
AVX support.

(Both the Intel and the AMD manuals are poor at explicitly listing
dependencies.  I have spent far too much time attempting to reverse
engineer the real dependency trees from the information provided.)

~Andrew

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