kvm: enable CPUID faulting on all VCPUs #11492
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This feature is controlled by an MSR; MSRs are per-CPU.
The Intel SDM doesn't document CPUID faulting, at least as of the Dec 2024
revision; despite the deleted comment in ring0/kernel_amd64.go, there is no
Vol. 3 Table 2-43, and every table in Vol. 4 ("Model-Specific Registers") lists
bit 31 in MSR_PLATFORM_INFO as "reserved". The only documentation seems to be
that cited by Linux's e9ea1e7f53b85 ("x86/arch_prctl: Add
ARCH_[GET|SET]_CPUID"): "Intel Virtualization Technology FlexMigration
Application Note" 323850-004, 2012. This document positions CPUID faulting as
an alternative way to support cross-CPU migration for VMs that don't use VMX;
consequently it does not clarify if CPUID faulting is effective in guest ("VMX
non-root") mode, or if the CPUID VM exit takes precedence. If the former is the
case then CPUID faulting is probably faster than setting app CPUID with
KVM_SET_CPUID2, and vice versa. But regardless, this is much simpler.
PiperOrigin-RevId: 733113944
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kvm: enable CPUID faulting on all VCPUs
This feature is controlled by an MSR; MSRs are per-CPU.
The Intel SDM doesn't document CPUID faulting, at least as of the Dec 2024
revision; despite the deleted comment in ring0/kernel_amd64.go, there is no
Vol. 3 Table 2-43, and every table in Vol. 4 ("Model-Specific Registers") lists
bit 31 in MSR_PLATFORM_INFO as "reserved". The only documentation seems to be
that cited by Linux's e9ea1e7f53b85 ("x86/arch_prctl: Add
ARCH_[GET|SET]_CPUID"): "Intel Virtualization Technology FlexMigration
Application Note" 323850-004, 2012. This document positions CPUID faulting as
an alternative way to support cross-CPU migration for VMs that don't use VMX;
consequently it does not clarify if CPUID faulting is effective in guest ("VMX
non-root") mode, or if the CPUID VM exit takes precedence. If the former is the
case then CPUID faulting is probably faster than setting app CPUID with
KVM_SET_CPUID2, and vice versa. But regardless, this is much simpler.