Check extended CPU features only when necessary (#426)

When checking for CPU features, we support some features under `get_extended_feature_info`, CPUID leaf `eax=7`. Unlike the other leaves we check (`eax=1` and `eax=80000001`), this leaf is not ubiquitous and a rough estimate from feature flags (specifically, `smep`, which Intel added in Ivy Bridge) suggests this leaf may not exist on processors from before 2012. Only check this leaf if the module we would load requires features reported by it, for compatibility with such processors.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1615786, wherein the machine's processor is an AMD Phenom II, a series that shipped in 2010.

Author: shravanrn

lucet-runtime/lucet-runtime-internals/src/module/dl.rs

@@ -48,24 +48,28 @@ fn check_feature_support(module_features: &ModuleFeatures) -> Result<(), Error>
         return Err(missing_feature("POPCNT"));
     }
 
-    let info = cpuid.get_extended_feature_info().ok_or_else(|| {
-        Error::Unsupported("Unable to obtain host CPU extended feature info!".to_string())
-    })?;
+    if module_features.bmi1 || module_features.bmi2 {
+        let info = cpuid.get_extended_feature_info().ok_or_else(|| {
+            Error::Unsupported("Unable to obtain host CPU extended feature info!".to_string())
+        })?;
 
-    if module_features.bmi1 && !info.has_bmi1() {
-        return Err(missing_feature("BMI1"));
-    }
+        if module_features.bmi1 && !info.has_bmi1() {
+            return Err(missing_feature("BMI1"));
+        }
 
-    if module_features.bmi2 && !info.has_bmi2() {
-        return Err(missing_feature("BMI2"));
+        if module_features.bmi2 && !info.has_bmi2() {
+            return Err(missing_feature("BMI2"));
+        }
     }
 
-    let info = cpuid.get_extended_function_info().ok_or_else(|| {
-        Error::Unsupported("Unable to obtain host CPU extended function info!".to_string())
-    })?;
+    if module_features.lzcnt {
+        let info = cpuid.get_extended_function_info().ok_or_else(|| {
+            Error::Unsupported("Unable to obtain host CPU extended function info!".to_string())
+        })?;
 
-    if module_features.lzcnt && !info.has_lzcnt() {
-        return Err(missing_feature("LZCNT"));
+        if module_features.lzcnt && !info.has_lzcnt() {
+            return Err(missing_feature("LZCNT"));
+        }
     }
 
     // Features are fine, we're compatible!