On Mon, 2016-10-31 at 10:53 +0100, Borislav Petkov wrote: > > I think that in v4.9-rc2 the CPUID[7,0].edx bits can be masked out > by > > applying noxsave to cmdline. Using directly cpu_count will result > in > > passing the bits in edx to a guest directly while the xsaveopt and > rest > > of AVX512 features bits will be cleared. > > Errr, I can't parse that reading it backwards and forwards. Please > elaborate.
The patch that introduces AVX512_4VNNIW and AVX512_4FMAPS features was merged to kernel 4.9-rc2 so we have possibility to mask the feature bits using 'noxsave' option in kernel cmdline. This option clears all AVX512 feature bits in boot_cpu_data.x86_capability. The cpuid_mask function, which usually used in kvm, read bit from this x86_capabity and mask out. This prevents passing disabled features to guest. If we use cpu_count instead, which reports bits directly from CPU, then the bits of features that are disabled in host are passed to guest as enabled. This seems be inconsistent. Thanks, Piotr
