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 12:10, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 30/11/15 11:08, Jan Beulich wrote:
>>>>> On 30.11.15 at 11:46, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> 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.
>> Yet I don't think "VEX encoding" == "AVX". See the various VEX-
>> encoded non-SIMD 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.
>> I.e. a dependency on XSAVE, but not on AVX.
> 
> XCR0[2:1] are the AVX and SSE bits.

You mean the YMM and XMM ones (the latter one is  mis-named in
our code base). 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). The minimal (and imo more natural)
dependency is just the XSTATE bit.

Jan


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