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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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