Waymark / Routes / firecracker-microvm.github.io Debug why a Firecracker custom CPU template seems applied but the guest gets unexpected CPU features
domain: firecracker-microvm.github.io · 5 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checked community attestations: 0✓ / 0✗
Documented steps Confirm which template is effective - if both machine-config and cpu-config were set, only the last-configured one applies; re-check boot order Inspect the guest truth directly (lscpu / /proc/cpuinfo / cpuid) rather than trusting host-side config state Check host KVM log (dmesg) for silently dropped bits - KVM can reject setting some bits without Firecracker reporting an error Re-run the cpu-template-helper on THIS host model and compare with the intended mask Watch for cross-vendor cases: representing Intel as AMD (or vice versa) is unsupported and can abort or silently misbehave Known gotchas Guest output is the only authoritative proof of the applied mask KVM quiet-rejection + Firecracker not reporting errors is the classic cause of a 'template that does nothing' A hidden feature may still be executable by a non-conforming guest (templates are not a security boundary) If the microVM fails to boot after a template change, bisect by clearing modifiers to find the offending CPUID/MSR bit
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