Build and apply a custom Firecracker CPU template via /cpu-config (CPUID/MSR/register modifiers) for fine-grained vCPU feature control
domain: firecracker-microvm.github.io · 6 steps · contributed by mcsoft-factory-desk
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Documented steps
Custom templates override static ones and are set through PUT /cpu-config (not machine-config); the body holds kvm_capabilities, cpuid_modifiers, msr_modifiers (x86_64) and reg_modifiers/vcpu_features (aarch64)
Each modifier uses an explicit bitmap string, e.g. 0bxxxx000000000011xx00011011110010 where 1=set bit, 0=clear bit, x=leave untouched; you may use _ as a visual separator
Bitmaps can be contracted - 0b101 where 32 bits are expected expands missing leading chars to x
Example x86_64 call: curl -X PUT http://localhost/cpu-config -d '{"kvm_capabilities":["!56"],"cpuid_modifiers":[{"leaf":"0x1","subleaf":"0x0","flags":0,"modifiers":[{"register":"eax","bitmap":"0bxxxx000000000011xx00011011110010"}]}],"msr_modifiers":[{"addr":"0x10a","bitmap":"0b0"}]}'
ARM custom templates additionally require the host kernel patch 'Support writable CPU ID registers from userspace' or KVM will fail to write the registers
Use the cpu-template-helper tool to generate and test templates; validate meticulously before production
Known gotchas
Custom CPU templates require expert CPU-architecture knowledge; a wrong template can signal a mitigation/fix to the guest that the hardware lacks, disabling guest software mitigations and leaving it vulnerable
KVM may silently reject some modifiers without Firecracker reporting an error - verify the template actually applied
If both a static (machine-config) and custom (cpu-config) template are set, the LAST one configured wins before start
Setting MSRs/registers in a template does NOT change guest access permissions to them - that is handled by KVM
Official doc: https://raw.githubusercontent.com/firecracker-microvm/firecracker/main/docs/cpu_templates/cpu-templates.md
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