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cpuid-normalization.md

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CPUID normalization (x86_64 only)

On x86_64, Firecracker makes certain modifications to the guest's CPUID regardless of whether a CPU template is used. This is referred to as CPUID normalization. If a CPU template is used the normalization is performed after the CPU template is applied. That means that if the CPU template configures CPUID bits used in the normalization process, they will be overwritten.

See also: boot protocol settings

x86_64 common CPUID normalization

Description Leaf Subleaf Register Bits
Set CLFLUSH line size 0x1 - EBX 15:8
Set maximum number of addressable IDs for logical processors in the physical package 0x1 - EBX 23:16
Set initial APIC ID 0x1 - EBX 31:24
Disable PDCM (Perfmon and Debug Capability) 0x1 - ECX 15
Enable TSC_DEADLINE 0x1 - ECX 24
Enable HYPERVISOR 0x1 - ECX 31
Set HTT value if the microVM's CPU count is greater than 1 0x1 - EDX 28
Insert leaf 0xb, subleaf 0x1 filled with 0 if it is not already present 0xb 0x1 all all
Update extended topology enumeration 0xb all EAX 4:0
Update extended topology enumeration 0xb all EBX 15:0
Update extended topology enumeration 0xb all ECX 15:8

Intel-specific CPUID normalization

Description Leaf Subleaf Register Bits
Update deterministic cache parameters 0x4 all EAX 31:14
Disable Intel Turbo Boost technology 0x6 - EAX 1
Disable frequency selection 0x6 - ECX 3
Disable performance monitoring 0xa - EAX, EBX, ECX, EDX all
Update brand string to use a default format and real frequency 0x80000002, 0x80000003, 0x80000004 - EAX, EBX, ECX, EDX all

AMD-specifc CPUID normalization

Description Leaf Subleaf Register Bits
Set IA32_ARCH_CAPABILITIES MSR as not present 0x7 - EDX 29
Update largest extended function entry to 0x8000001f 0x80000000 - EAX 31:0
Set topology extension bit 0x80000001 - ECX 22
Update brand string with a default AMD value 0x80000002, 0x80000003, 0x80000004 - EAX, EBX, ECX, EDX all
Update number of physical threads 0x80000008 - ECX 7:0
Update APIC ID size 0x80000008 - ECX 15:12
Update cache topology information 0x8000001d all all all
Update extended APIC ID 0x8000001e - EAX, EBX, ECX all