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vmm-reference-vcpuid.md

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cpudid.rs

CPUID instruction in x86

Can be used by the software to determine the type of CPU it is running on and what features it provides.

The instruction does not have an explicit parameter. Instead EAX must be set before calling the instruction to indicate a function. The information returned in stored in the EBX, EDX, ECX registers.

Some examples of functions are:

EAX = 0: Stores the manufacturer's id as ASCII string in the registers EBX, EDX, ECX and sets EAX to be highest calling parameter value that the CPU supports.
Common common manufacturer id values are:

"GenuineIntel": Intel "AuthenticAMD": AMD "AMDisbetter!": early AMD chips

"KVMKVMKVM ": KVM "VMwareVMware": VMware

EAX = 1:

returns the family and model of the CPU in EAX.
returns the features supported by the cpu such as hyper-threading, vmx, debug instructions, lapic, simd etc..

EAX = 2: Cache and TLB Description

EAX = 6: Power & Thermal Management features

For a VM most of the cpu id information is set by KVM.

The file cpuid.rs contains a function filter_cpuid the overrides some of the values in used by the default KVM. The top of the file contains the positions of CPUID bits for specific information that the filter function may override.

This function is used in the file vcpu/mod.rs while creating config for the vcpus of a vm.

        for index in 0..num_vcpus {
            // Set CPUID.
            #[cfg(target_arch = "x86_64")]
            let mut cpuid = base_cpuid.clone();
            #[cfg(target_arch = "x86_64")]
            vm_vcpu_ref::x86_64::cpuid::filter_cpuid(_kvm, index, num_vcpus, &mut cpuid);

            #[cfg(target_arch = "x86_64")]
            let vcpu_config = VcpuConfig {
                cpuid,
                id: index,
                msrs: supported_msrs.clone(),
            };

The config is then used in a KVM_SET_CPUID call before the cpu is run.
Whenever the vCPU calls the CPUDID instruction, a VM_EXIT happens and it is handled by the KVM kernel exit handler.