[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] x86/PV: hide features dependent on XSAVE when booted with "no-xsave"
>>> 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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