[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 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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