Messages in this thread | | | From | Vladimir Davydov <> | Date | Fri, 20 Jul 2012 22:21:58 +0400 | Subject | Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features |
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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 > >
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