illumos gate

Presently, bhyve exposes feature bits through cpuid solely by applying static masks against the corresponding leafs from the host CPU. While this does work, it poses a challenge for stabilizing the features exposed via cpuid, especially in the face of guests migrated between machines (potentially running different hardware or software). In order to give userspace more control over cpuid emulation, while still maintaining the relative performance of in-kernel emulation for it, bhyve should add functionality similar to KVM for setting/getting a list of leafs to be used for a given instance.

The structs/defines for KVM_SET_CPUID2 (the newer, preferred, interface) are as follows:

struct kvm_cpuid_entry2 {
        __u32 function;
        __u32 index;
        __u32 flags;
        __u32 eax;
        __u32 ebx;
        __u32 ecx;
        __u32 edx;
        __u32 padding[3];
};

#define KVM_CPUID_FLAG_SIGNIFCANT_INDEX         (1 << 0)
#define KVM_CPUID_FLAG_STATEFUL_FUNC            (1 << 1)
#define KVM_CPUID_FLAG_STATE_READ_NEXT          (1 << 2)

/* for KVM_SET_CPUID2 */
struct kvm_cpuid2 {
        __u32 nent;
        __u32 padding;
        struct kvm_cpuid_entry2 entries[0];
};

Since we don't use per-vcpu file descriptors, we'll probably want to shuffle our struct kvm_cpuid2 equivalent to have the vcpuid as the first member, with nent as the second, omitting the padding.