target/arm: Fix has_vfp/has_neon ID reg squashing for M-profile

In arm_cpu_realizefn(), if the CPU has VFP or Neon disabled then we
squash the ID register fields so that we don't advertise it to the
guest.  This code was written for A-profile and needs some tweaks to
work correctly on M-profile:

 * A-profile only fields should not be zeroed on M-profile:
   - MVFR0.FPSHVEC,FPTRAP
   - MVFR1.SIMDLS,SIMDINT,SIMDSP,SIMDHP
   - MVFR2.SIMDMISC
 * M-profile only fields should be zeroed on M-profile:
   - MVFR1.FP16

In particular, because MVFR1.SIMDHP on A-profile is the same field as
MVFR1.FP16 on M-profile this code was incorrectly disabling FP16
support on an M-profile CPU (where has_neon is always false).  This
isn't a visible bug yet because we don't have any M-profile CPUs with
FP16 support, but the change is necessary before we introduce any.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20201019151301.2046-9-peter.maydell@linaro.org

target/arm/cpu.c

@@ -1429,17 +1429,22 @@ static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
         u = cpu->isar.mvfr0;
         u = FIELD_DP32(u, MVFR0, FPSP, 0);
         u = FIELD_DP32(u, MVFR0, FPDP, 0);
-        u = FIELD_DP32(u, MVFR0, FPTRAP, 0);
         u = FIELD_DP32(u, MVFR0, FPDIVIDE, 0);
         u = FIELD_DP32(u, MVFR0, FPSQRT, 0);
-        u = FIELD_DP32(u, MVFR0, FPSHVEC, 0);
         u = FIELD_DP32(u, MVFR0, FPROUND, 0);
+        if (!arm_feature(env, ARM_FEATURE_M)) {
+            u = FIELD_DP32(u, MVFR0, FPTRAP, 0);
+            u = FIELD_DP32(u, MVFR0, FPSHVEC, 0);
+        }
         cpu->isar.mvfr0 = u;
 
         u = cpu->isar.mvfr1;
         u = FIELD_DP32(u, MVFR1, FPFTZ, 0);
         u = FIELD_DP32(u, MVFR1, FPDNAN, 0);
         u = FIELD_DP32(u, MVFR1, FPHP, 0);
+        if (arm_feature(env, ARM_FEATURE_M)) {
+            u = FIELD_DP32(u, MVFR1, FP16, 0);
+        }
         cpu->isar.mvfr1 = u;
 
         u = cpu->isar.mvfr2;
@@ -1475,16 +1480,18 @@ static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
         u = FIELD_DP32(u, ID_ISAR6, FHM, 0);
         cpu->isar.id_isar6 = u;
 
-        u = cpu->isar.mvfr1;
-        u = FIELD_DP32(u, MVFR1, SIMDLS, 0);
-        u = FIELD_DP32(u, MVFR1, SIMDINT, 0);
-        u = FIELD_DP32(u, MVFR1, SIMDSP, 0);
-        u = FIELD_DP32(u, MVFR1, SIMDHP, 0);
-        cpu->isar.mvfr1 = u;
-
-        u = cpu->isar.mvfr2;
-        u = FIELD_DP32(u, MVFR2, SIMDMISC, 0);
-        cpu->isar.mvfr2 = u;
+        if (!arm_feature(env, ARM_FEATURE_M)) {
+            u = cpu->isar.mvfr1;
+            u = FIELD_DP32(u, MVFR1, SIMDLS, 0);
+            u = FIELD_DP32(u, MVFR1, SIMDINT, 0);
+            u = FIELD_DP32(u, MVFR1, SIMDSP, 0);
+            u = FIELD_DP32(u, MVFR1, SIMDHP, 0);
+            cpu->isar.mvfr1 = u;
+
+            u = cpu->isar.mvfr2;
+            u = FIELD_DP32(u, MVFR2, SIMDMISC, 0);
+            cpu->isar.mvfr2 = u;
+        }
     }
 
     if (!cpu->has_neon && !cpu->has_vfp) {