[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 15:22, Jan Beulich wrote: >>>> 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. What? That is absurd. They depend on AVX. I am not introducing a synthetic feature when the real feature bits are correct. ~Andrew _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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