[Qemu-devel] [PATCH v2] target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

[Thomas Huth <thuth@redhat.com>]

Currently we only present the plain z900 feature bits to the guest,
but QEMU already emulates some additional features (but not all of
the next CPU generation, so we can not use the next CPU level as
default yet). Since newer Linux kernels are checking the feature bits
and refuse to work if a required feature is missing, it would be nice
to have a way to present more of the supported features when we are
running with the "qemu" CPU.
This patch now adds the supported features to the "full_feat" bitmap,
so that additional features can be enabled on the command line now,
for example with:

 qemu-system-s390x -cpu qemu,stfle=true,ldisp=true,eimm=true,stckf=true

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v2: Mark feats array with "static const"

 target/s390x/cpu_models.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
index 8d27363..e5e005a 100644
--- a/target/s390x/cpu_models.c
+++ b/target/s390x/cpu_models.c
@@ -658,6 +658,30 @@ static void check_compatibility(const S390CPUModel *max_model,
                   "available in the configuration: ");
 }
 
+/**
+ * The base TCG CPU model "qemu" is based on the z900. However, we already
+ * can also emulate some additional features of later CPU generations, so
+ * we add these additional feature bits here.
+ */
+static void add_qemu_cpu_model_features(S390FeatBitmap fbm)
+{
+    static const int feats[] = {
+        S390_FEAT_STFLE,
+        S390_FEAT_EXTENDED_IMMEDIATE,
+        S390_FEAT_LONG_DISPLACEMENT,
+        S390_FEAT_LONG_DISPLACEMENT_FAST,
+        S390_FEAT_STORE_CLOCK_FAST,
+        S390_FEAT_GENERAL_INSTRUCTIONS_EXT,
+        S390_FEAT_EXECUTE_EXT,
+        S390_FEAT_STFLE_45,
+    };
+    int i;
+
+    for (i = 0; i < ARRAY_SIZE(feats); i++) {
+        set_bit(feats[i], fbm);
+    }
+}
+
 static S390CPUModel *get_max_cpu_model(Error **errp)
 {
     static S390CPUModel max_model;
@@ -670,10 +694,11 @@ static S390CPUModel *get_max_cpu_model(Error **errp)
     if (kvm_enabled()) {
         kvm_s390_get_host_cpu_model(&max_model, errp);
     } else {
-        /* TCG emulates a z900 */
+        /* TCG emulates a z900 (with some optional additional features) */
         max_model.def = &s390_cpu_defs[0];
         bitmap_copy(max_model.features, max_model.def->default_feat,
                     S390_FEAT_MAX);
+        add_qemu_cpu_model_features(max_model.features);
     }
     if (!*errp) {
         cached = true;
@@ -925,11 +950,14 @@ static void s390_host_cpu_model_initfn(Object *obj)
 
 static void s390_qemu_cpu_model_initfn(Object *obj)
 {
+    static S390CPUDef s390_qemu_cpu_defs;
     S390CPU *cpu = S390_CPU(obj);
 
     cpu->model = g_malloc0(sizeof(*cpu->model));
-    /* TCG emulates a z900 */
-    cpu->model->def = &s390_cpu_defs[0];
+    /* TCG emulates a z900 (with some optional additional features) */
+    memcpy(&s390_qemu_cpu_defs, &s390_cpu_defs[0], sizeof(s390_qemu_cpu_defs));
+    add_qemu_cpu_model_features(s390_qemu_cpu_defs.full_feat);
+    cpu->model->def = &s390_qemu_cpu_defs;
     bitmap_copy(cpu->model->features, cpu->model->def->default_feat,
                 S390_FEAT_MAX);
 }
-- 
1.8.3.1