[RFC PATCH 1/2] hw/i386: -cpu model, -feature, +feature should enable feature

[David Edmondson <david.edmondson@oracle.com>]

"Minus" features are applied after "plus" features, so ensure that a
later "plus" feature causes an earlier "minus" feature to be removed.

This has no effect on the existing "-feature,feature=on" backward
compatibility code (which warns and turns the feature off).

Signed-off-by: David Edmondson <david.edmondson@oracle.com>
---
 target/i386/cpu.c                   | 33 +++++++++++++++++++++++------
 tests/qtest/test-x86-cpuid-compat.c |  8 +++----
 2 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 35459a38bb..13f58ef183 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -4750,13 +4750,32 @@ static void x86_cpu_parse_featurestr(const char *typename, char *features,
         GlobalProperty *prop;
 
         /* Compatibility syntax: */
-        if (featurestr[0] == '+') {
-            plus_features = g_list_append(plus_features,
-                                          g_strdup(featurestr + 1));
-            continue;
-        } else if (featurestr[0] == '-') {
-            minus_features = g_list_append(minus_features,
-                                           g_strdup(featurestr + 1));
+        if (featurestr[0] == '+' || featurestr[0] == '-') {
+            const char *feat = featurestr + 1;
+            GList **remove, **add;
+            GList *val;
+
+            if (featurestr[0] == '+') {
+                remove = &minus_features;
+                add = &plus_features;
+            } else {
+                remove = &plus_features;
+                add = &minus_features;
+            }
+
+            val = g_list_find_custom(*remove, feat, compare_string);
+            if (val) {
+                char *data = val->data;
+
+                *remove = g_list_remove(*remove, data);
+                g_free(data);
+            }
+
+            val = g_list_find_custom(*add, feat, compare_string);
+            if (!val) {
+                *add = g_list_append(*add, g_strdup(feat));
+            }
+
             continue;
         }
 
diff --git a/tests/qtest/test-x86-cpuid-compat.c b/tests/qtest/test-x86-cpuid-compat.c
index 7ca1883a29..6824d2b13e 100644
--- a/tests/qtest/test-x86-cpuid-compat.c
+++ b/tests/qtest/test-x86-cpuid-compat.c
@@ -171,18 +171,18 @@ static void test_plus_minus_subprocess(void)
     char *path;
 
     /* Rules:
-     * 1)"-foo" overrides "+foo"
+     * 1) The later of "+foo" or "-foo" wins
      * 2) "[+-]foo" overrides "foo=..."
      * 3) Old feature names with underscores (e.g. "sse4_2")
      *    should keep working
      *
-     * Note: rules 1 and 2 are planned to be removed soon, and
-     * should generate a warning.
+     * Note: rule 2 is planned to be removed soon, and should generate
+     * a warning.
      */
     qtest_start("-cpu pentium,-fpu,+fpu,-mce,mce=on,+cx8,cx8=off,+sse4_1,sse4_2=on");
     path = get_cpu0_qom_path();
 
-    g_assert_false(qom_get_bool(path, "fpu"));
+    g_assert_true(qom_get_bool(path, "fpu"));
     g_assert_false(qom_get_bool(path, "mce"));
     g_assert_true(qom_get_bool(path, "cx8"));
 
-- 
2.29.2