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.

x86_64 common CPUID normalization

Description	Leaf	Subleaf	Register	Bits
Pass through vendor ID from host	0x0	-	EBX, ECX, EDX	all
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 if not present	0xb	0x1	all	all
Fill extended topology enumeration leaf	0xb	all	all	all
Pass through L1 cache and TLB information from host	0x80000005	-	all	all
Pass through L2 cache and TLB and L3 cache information from host	0x80000006	-	all	all

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
Set FDP_EXCPTN_ONLY bit	0x7	0x0	EBX	6
Set "Deprecates FPU CS and FPU DS values" bit	0x7	0x0	EBX	13
Disable WAITPKG (UMONITOR / UMWAIT / TPAUSE)	0x7	0x0	ECX	5
Disable performance monitoring	0xa	-	all	all
Fill v2 extended topology enumeration leaf	0x1f	all	all	all
Update brand string to use a default format and real frequency	0x80000002, 0x80000003, 0x80000004	-	all	all

AMD-specifc CPUID normalization

Description	Leaf	Subleaf	Register	Bits
Set IA32_ARCH_CAPABILITIES MSR as not present	0x7	-	EDX	29
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