target/arm/cpu64: Ensure kvm really supports aarch64=off

If -cpu <cpu>,aarch64=off is used then KVM must also be used, and it
and the host must support running the vcpu in 32-bit mode. Also, if
-cpu <cpu>,aarch64=on is used, then it doesn't matter if kvm is
enabled or not.
Signed-off-by: NAndrew Jones <drjones@redhat.com>
Reviewed-by: NEric Auger <eric.auger@redhat.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target/arm/cpu64.c

@@ -426,13 +426,13 @@ static void aarch64_cpu_set_aarch64(Object *obj, bool value, Error **errp)
      * restriction allows us to avoid fixing up functionality that assumes a
      * uniform execution state like do_interrupt.
      */
-    if (!kvm_enabled()) {
-        error_setg(errp, "'aarch64' feature cannot be disabled "
-                         "unless KVM is enabled");
-        return;
-    }
-
     if (value == false) {
+        if (!kvm_enabled() || !kvm_arm_aarch32_supported(CPU(cpu))) {
+            error_setg(errp, "'aarch64' feature cannot be disabled "
+                             "unless KVM is enabled and 32-bit EL1 "
+                             "is supported");
+            return;
+        }
         unset_feature(&cpu->env, ARM_FEATURE_AARCH64);
     } else {
         set_feature(&cpu->env, ARM_FEATURE_AARCH64);

target/arm/kvm64.c

@@ -24,7 +24,9 @@
 #include "exec/gdbstub.h"
 #include "sysemu/sysemu.h"
 #include "sysemu/kvm.h"
+#include "sysemu/kvm_int.h"
 #include "kvm_arm.h"
+#include "hw/boards.h"
 #include "internals.h"
 
 static bool have_guest_debug;
@@ -593,6 +595,13 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf)
     return true;
 }
 
+bool kvm_arm_aarch32_supported(CPUState *cpu)
+{
+    KVMState *s = KVM_STATE(current_machine->accelerator);
+
+    return kvm_check_extension(s, KVM_CAP_ARM_EL1_32BIT);
+}
+
 #define ARM_CPU_ID_MPIDR       3, 0, 0, 0, 5
 
 int kvm_arch_init_vcpu(CPUState *cs)

target/arm/kvm_arm.h

@@ -207,6 +207,15 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf);
  */
 void kvm_arm_set_cpu_features_from_host(ARMCPU *cpu);
 
+/**
+ * kvm_arm_aarch32_supported:
+ * @cs: CPUState
+ *
+ * Returns: true if the KVM VCPU can enable AArch32 mode
+ * and false otherwise.
+ */
+bool kvm_arm_aarch32_supported(CPUState *cs);
+
 /**
  * kvm_arm_get_max_vm_ipa_size - Returns the number of bits in the
  * IPA address space supported by KVM
@@ -247,6 +256,11 @@ static inline void kvm_arm_set_cpu_features_from_host(ARMCPU *cpu)
     cpu->host_cpu_probe_failed = true;
 }
 
+static inline bool kvm_arm_aarch32_supported(CPUState *cs)
+{
+    return false;
+}
+
 static inline int kvm_arm_get_max_vm_ipa_size(MachineState *ms)
 {
     return -ENOENT;