x86/gen-cpuid: correct cycle detection

With the processing done linearly (rather than recursively), checking
whether any of the features was previously seen is wrong: That would
e.g. trigger for this simple set of dependencies

    X: [A, B]
    A: [C]
    B: [C]

(observed in reality when making AMX-AVX512 dependent upon both
AMX-TILE and AVX512F, causing XSAVE to see AMX-AVX512 twice in its list
of dependents). But checking the whole accumulated set also isn't
necessary - just checking the feature we're processing dependents of is
sufficient. We may detect a cycle later that way, but we still will
detect it. What we need to avoid is adding a feature again when we've
already seen it.

As a result, seeding "seen[]" with "feat" isn't necessary anymore.

Fixes: fe4408d180 ("xen/x86: Generate deep dependencies of features")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 3999ff0d30
master date: 2025-09-01 13:07:53 +0200

xen/tools/gen-cpuid.py

@@ -350,7 +350,7 @@ def crunch_numbers(state):
 
     for feat in deep_features:
 
-        seen = [feat]
+        seen = []
         to_process = list(deps[feat])
 
         while len(to_process):
@@ -363,14 +363,17 @@ def crunch_numbers(state):
 
             f = to_process.pop(0)
 
+            if f == feat:
+                raise Fail("ERROR: Cycle found when processing %s" %
+                           (state.names[f], ))
+
             if f in seen:
-                raise Fail("ERROR: Cycle found with %s when processing %s"
-                           % (state.names[f], state.names[feat]))
+                continue
 
             seen.append(f)
             to_process = list(set(to_process + deps.get(f, [])))
 
-        state.deep_deps[feat] = seen[1:]
+        state.deep_deps[feat] = seen
 
     state.deep_features = deps.keys()
     state.nr_deep_deps = len(state.deep_deps.keys())