Configure a custom Firecracker CPU template on aarch64 (ARM) using reg_modifiers and vcpu_features
domain: firecracker-microvm.github.io · 5 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Ensure the host kernel has the 'Support writable CPU ID registers from userspace' patch, otherwise KVM fails to write ARM registers
Build JSON with reg_modifiers (register addr + bitmap, e.g. addr '0x603000000013c020' with a 64-bit mask) and vcpu_features (index + bitmap, e.g. index 0 with '0b11xxxxx')
Add required kvm_capabilities such as KVM_CAP_ARM_PTRAUTH_ADDRESS (171) and KVM_CAP_ARM_PTRAUTH_GENERIC (172) to gate boot on the host supporting ptrauth
PUT /cpu-config before boot and start the microVM
Boot a guest under the same template across ARM hosts to confirm a consistent CPU feature set
Known gotchas
ARM CPU templates fail hard without the writable-CPU-ID-registers kernel patch
Arm register addresses are constructed per the KVM API documentation (docs.kernel.org/virt/kvm/api.html); wrong addresses silently no-op or fail
vCPU feature indices map to KVM_ARM_VCPU_* values from arch/arm64/include/uapi/asm/kvm.h
ARM template bitmaps are 64 bits; pad/contract with 'x' as needed
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