chore(vmm): Warn on unexpected subleaf for CPUID leaf 0xB

KVM no longer returns any subleaves greater than 0. The patch was merged
in v6.2 and backported to v5.10. So for all our supported kernels,
subleaves >= 2 should not be included [1].

However, we intentionally leave Firecracker not fail for unsupported
kernels to keep working. Note that we can detect KVM regression thanks
to the test that compares a fingerprint with its baseline.

So prints a warning message without bailing out.

[1]: torvalds/linux@45e966f

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

src/vmm/src/cpu_config/x86_64/cpuid/normalize.rs

@@ -4,6 +4,7 @@
 use crate::cpu_config::x86_64::cpuid::{
     CpuidEntry, CpuidKey, CpuidRegisters, CpuidTrait, KvmCpuidFlags, cpuid,
 };
+use crate::logger::warn;
 use crate::vmm_config::machine_config::MAX_SUPPORTED_VCPUS;
 
 /// Error type for [`super::Cpuid::normalize`].
@@ -370,8 +371,15 @@ impl super::Cpuid {
                             .map_err(|err| ExtendedTopologyError::DomainType(index, err))?;
                     }
                     _ => {
-                        // We expect here as this is an extremely rare case that is unlikely to ever
-                        // occur.
+                        // KVM no longer returns any subleaves greater than 0. The patch was merged
+                        // in v6.2 and backported to v5.10. So for all our supported kernels,
+                        // subleaves >= 2 should not be included.
+                        // https://github.com/torvalds/linux/commit/45e966fcca03ecdcccac7cb236e16eea38cc18af
+                        //
+                        // However, we intentionally leave Firecracker not fail for unsupported
+                        // kernels to keep working. Note that we can detect KVM regression thanks
+                        // to the test that compares a fingerprint with its baseline.
+                        warn!("Subleaf {index} not expected for CPUID leaf 0xB.");
                         subleaf.result.ecx = index;
                     }
                 }