Hi Florian, I understand you want to discuss the x86_64 micro-architecture levels only in this thread, but it would be nice to have a similar discussion for other architectures.
One thing that wasn't clear to me from this proposal is how the glibc dynamic loader checks for the CPU feature flags. This is important for valgrind since it can communicate those through different means. cpuid interception, auxv AT_HWCAP/AT_HWCAP2 interception (but not AT_PLATFORM at the moment) and of course we can generate SIGILL for unsupported instructions. We currently don't intercept /proc/cpuinfo (but could). I think it is important to be precise here, because in the past this has sometimes caused confusion. For example for how to check correctly for avx, lzcnt, or fma[4] support. Thanks, Mark P.S. I don't particular like the numbered names, but well, bike-shed...
