[Qemu-devel] [PATCH 2/7] target/i386: introduce generic feature dependency mechanism

[Paolo Bonzini <pbonzini@redhat.com>]

Sometimes a CPU feature does not make sense unless another is
present.  In the case of VMX features, KVM does not even allow
setting the VMX controls to some invalid combinations.

Therefore, this patch adds a generic mechanism that looks for bits
that the user explicitly cleared, and uses them to remove other bits
from the expanded CPU definition.  If these dependent bits were also
explicitly *set* by the user, this will be a warning for "-cpu check"
and an error for "-cpu enforce".  If not, then the dependent bits are
cleared silently, for convenience.

With VMX features, this will be used so that for example
"-cpu host,-rdrand" will also hide support for RDRAND exiting.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/cpu.c | 77 +++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 49 insertions(+), 28 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 9149d0d..412e834 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -799,10 +799,6 @@ typedef struct FeatureWordInfo {
         /* If type==MSR_FEATURE_WORD */
         struct {
             uint32_t index;
-            struct {   /*CPUID that enumerate this MSR*/
-                FeatureWord cpuid_class;
-                uint32_t    cpuid_flag;
-            } cpuid_dep;
         } msr;
     };
     uint32_t tcg_features; /* Feature flags supported by TCG */
@@ -1197,10 +1193,6 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
         },
         .msr = {
             .index = MSR_IA32_ARCH_CAPABILITIES,
-            .cpuid_dep = {
-                FEAT_7_0_EDX,
-                CPUID_7_0_EDX_ARCH_CAPABILITIES
-            }
         },
     },
     [FEAT_CORE_CAPABILITY] = {
@@ -1217,14 +1209,26 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
         },
         .msr = {
             .index = MSR_IA32_CORE_CAPABILITY,
-            .cpuid_dep = {
-                FEAT_7_0_EDX,
-                CPUID_7_0_EDX_CORE_CAPABILITY,
-            },
         },
     },
 };
 
+typedef struct FeatureDep {
+    uint16_t from, to;
+    uint64_t from_flag, to_flags;
+} FeatureDep;
+
+static FeatureDep feature_dependencies[] = {
+    {
+        .from = FEAT_7_0_EDX,            .from_flag = CPUID_7_0_EDX_ARCH_CAPABILITIES,
+        .to = FEAT_ARCH_CAPABILITIES,    .to_flags = ~0ull,
+    },
+    {
+        .from = FEAT_7_0_EDX,            .from_flag = CPUID_7_0_EDX_CORE_CAPABILITY,
+        .to = FEAT_CORE_CAPABILITY,      .to_flags = ~0ull,
+    },
+};
+
 typedef struct X86RegisterInfo32 {
     /* Name of register */
     const char *name;
@@ -5086,9 +5090,42 @@ static void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
 {
     CPUX86State *env = &cpu->env;
     FeatureWord w;
+    int i;
     GList *l;
     Error *local_err = NULL;
 
+    for (l = plus_features; l; l = l->next) {
+        const char *prop = l->data;
+        object_property_set_bool(OBJECT(cpu), true, prop, &local_err);
+        if (local_err) {
+            goto out;
+        }
+    }
+
+    for (l = minus_features; l; l = l->next) {
+        const char *prop = l->data;
+        object_property_set_bool(OBJECT(cpu), false, prop, &local_err);
+        if (local_err) {
+            goto out;
+        }
+    }
+
+    for (i = 0; i < ARRAY_SIZE(feature_dependencies); i++) {
+        FeatureDep *d = &feature_dependencies[i];
+        if ((env->user_features[d->from] & d->from_flag) &&
+            !(env->features[d->from] & d->from_flag)) {
+            uint64_t unavailable_features = env->features[d->to] & d->to_flags;
+
+            /* Not an error unless the dependent feature was added explicitly.  */
+            mark_unavailable_features(cpu, d->to, unavailable_features & env->user_features[d->to],
+                                      "This feature depends on other features that were not requested");
+
+            /* Prevent adding the feature in the loop below.  */
+            env->user_features[d->to] |= d->to_flags;
+            env->features[d->to] &= ~d->to_flags;
+        }
+    }
+
     /*TODO: Now cpu->max_features doesn't overwrite features
      * set using QOM properties, and we can convert
      * plus_features & minus_features to global properties
@@ -5106,22 +5143,6 @@ static void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
         }
     }
 
-    for (l = plus_features; l; l = l->next) {
-        const char *prop = l->data;
-        object_property_set_bool(OBJECT(cpu), true, prop, &local_err);
-        if (local_err) {
-            goto out;
-        }
-    }
-
-    for (l = minus_features; l; l = l->next) {
-        const char *prop = l->data;
-        object_property_set_bool(OBJECT(cpu), false, prop, &local_err);
-        if (local_err) {
-            goto out;
-        }
-    }
-
     if (!kvm_enabled() || !cpu->expose_kvm) {
         env->features[FEAT_KVM] = 0;
     }
-- 
1.8.3.1