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From: Kyle Huey <me@kylehuey.com>
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Subject: [PATCH v9 0/7] x86/arch_prctl Add ARCH_[GET|SET]_CPUID for controlling the CPUID instruction
Date: Sun,  6 Nov 2016 12:57:35 -0800
Message-Id: <20161106205742.4042-1-khuey@kylehuey.com>
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rr (http://rr-project.org/), a userspace record-and-replay reverse-
execution debugger, would like to trap and emulate the CPUID instruction.
This would allow us to a) mask away certain hardware features that rr does
not support (e.g. RDRAND) and b) enable trace portability across machines
by providing constant results.

Newer Intel CPUs (Ivy Bridge and later) can fault when CPUID is executed at
CPL > 0. Expose this capability to userspace as a new pair of arch_prctls,
ARCH_GET_CPUID and ARCH_SET_CPUID, with two values, ARCH_CPUID_ENABLE and
ARCH_CPUID_SIGSEGV.

Since v8:
Patch 7: KVM: x86: virtualize cpuid faulting
- Moved vcpu->arch.cpuid_fault initialization to the '!init_event' block.
- Added support for hypervisors writing to MSR_PLATFORM_INFO to disable the
  cpuid faulting feature.
- Added MSR_MISC_FEATURES_ENABLES to emulated_msrs[] to allow hypervisors
  to save/restore the msr.