Il 19/09/2014 09:58, Borislav Petkov ha scritto: >> > The trivial example is feature bits like XSAVE. We query them all the >> > time without checking the family when they were first introduced, >> > don't we? > The feature bits would obviously be 0 if features are not supported. And similarly, Intel would not extend a bit from 16 to 17 bits if it weren't zero on all older processors. > However, even there > > "16 - Reserved - Do not count on the value." > > I'm quoting Intel's CPUID doc 241618-037 from 2011 (there might be a > newer one though), the CPUID(1).ECX description. Once that bit gets a meaning in newer processors, the same meaning will work retroactively for existing processors. That's just how CPUID is used. Nobody checks families before testing bits, Intel/AMD do not even suggest that. > Do you have a guarantee that this won't happen in the future and break > all your fancy bitfields assumptions? No guarantee, but were that to happen, I'd expect tar and feathers spectacles around Intel's engineering headquarters. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
