The ARM extensions configuration *must* match the existing vCPUs already
initialized in KVM at VM initialization. ARM does not allow any per-vCPU
features to be changed once the system has fully initialized. This is an
immutable constraint of the ARM CPU architecture.

Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
---
 target/arm/cpu.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 3de0cb346b..14fcabc2c9 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -1912,6 +1912,49 @@ static void arm_cpu_finalizefn(Object *obj)
 #endif
 }
 
+static void arm_cpu_check_features_change(ARMCPU *cpu, Error **errp)
+{
+#if defined(TARGET_AARCH64) && !defined(CONFIG_USER_ONLY)
+    MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
+    ARMCPU *firstcpu = ARM_CPU(first_cpu);
+    DeviceState *dev = DEVICE(cpu);
+
+    if (!arm_feature(&cpu->env, ARM_FEATURE_AARCH64)) {
+        return;
+    }
+
+    /* For now, features of hotplugged CPU MUST match earlier booted CPUs */
+    if (!dev->hotplugged || !mc->has_hotpluggable_cpus) {
+        return;
+    }
+
+    if (cpu_isar_feature(aa64_sve, cpu) &&
+        (cpu->sve_max_vq != firstcpu->sve_max_vq ||
+         cpu->sve_vq.map != firstcpu->sve_vq.map)) {
+        error_setg(errp,
+                   "CPU %d: 'SVE' feature didn't match with existing CPUs",
+                   CPU(cpu)->cpu_index);
+        return;
+    }
+
+    if (cpu_isar_feature(aa64_sme, cpu) &&
+        (cpu->sme_vq.map != firstcpu->sme_vq.map)) {
+        error_setg(errp,
+                   "CPU %d: 'SME' feature didn't match with exisitng CPUs",
+                   CPU(cpu)->cpu_index);
+        return;
+    }
+
+    if (cpu_isar_feature(aa64_pauth, cpu) &&
+        (cpu->prop_pauth != firstcpu->prop_pauth)) {
+        error_setg(errp,
+                   "CPU %d: 'PAuth' feature didn't match with exisitng CPUs",
+                   CPU(cpu)->cpu_index);
+        return;
+    }
+#endif
+}
+
 void arm_cpu_finalize_features(ARMCPU *cpu, Error **errp)
 {
     Error *local_err = NULL;
@@ -1961,6 +2004,13 @@ void arm_cpu_finalize_features(ARMCPU *cpu, Error **errp)
             return;
         }
     }
+
+    /*
+     * As of now, we do not support heterogeneous computing, hence, features of
+     * all cpus should match. Hotplugged vCPUs are not allowed to have
+     * different features than the existing cold-plugged vCPUs
+     */
+    arm_cpu_check_features_change(cpu, &local_err);
 }
 
 static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
-- 
2.34.1