On Jul 20, 2012, at 9:20 PM, H. Peter Anvin wrote: > On 07/20/2012 09:37 AM, Vladimir Davydov wrote: >> If 'clearcpuid=N' is specified in boot options, CPU feature #N won't be >> reported in /proc/cpuinfo and used by the kernel. However, if a >> userpsace process checks CPU features directly using the cpuid >> instruction, it will be reported about all features supported by the CPU >> irrespective of what features are cleared. >> >> The patch makes the clearcpuid boot option not only clear CPU features >> in kernel but also mask them in hardware for Intel and AMD CPUs that >> support it so that the features cleared won't be reported even by the >> cpuid instruction. >> >> This can be useful for migration of virtual machines managed by >> hypervisors that do not support/use Intel VT/AMD-V hardware-assisted >> virtualization technology. >> >> If CPUID masking is supported, this will be reported in >> /proc/cpuinfo:flags as 'cpuidmask'. > > I am a bit concerned about this patch: > > 1. it silently changes existing behavior.
Yes, but who needs the current implementation of 'clearcpuid' which, in fact, just hides flags in /proc/cpuinfo while userspace apps will see and consequently use all CPU features? So, I think it logically extends the existing behavior. > 2. even on enabled hardware, only some of the bits are maskable. The patch makes only words 0, 1, 4, 6 maskable, but words 3, 7, 8 are Linux-defined, words 2 and 5 are Transmeta-, Centaur-, etc- defined, and word 9 contains some bizarre Intel CPU features. Thus, it is words 0, 1, 4, 6 that contain useful information for most hardware models. If you ask about some Intel CPUs that can't mask CPUID function 0x80000001, this function describes AMD-specific features, and I bet those Intel CPUs just don't have them at all and thus have nothing to mask. > > -hpa > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
