arm: Don't crash if user tries to use a Cortex-M CPU without an NVIC

The Cortex-M CPU and its NVIC are two intimately intertwined parts of
the same hardware; it is not possible to use one without the other.
Unfortunately a lot of our board models don't do any sanity checking
on the CPU type the user asks for, so a command line like
    qemu-system-arm -M versatilepb -cpu cortex-m3
will create an M3 without an NVIC, and coredump immediately.
In the other direction, trying a non-M-profile CPU in an M-profile
board won't blow up, but doesn't do anything useful either:
    qemu-system-arm -M lm3s6965evb -cpu arm926

Add some checking in the NVIC and CPU realize functions that the
user isn't trying to use an NVIC without an M-profile CPU or
an M-profile CPU without an NVIC, so we can produce a helpful
error message rather than a core dump.

Fixes: https://bugs.launchpad.net/qemu/+bug/1766896
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180601160355.15393-1-peter.maydell@linaro.org

hw/arm/armv7m.c

@@ -178,6 +178,12 @@ static void armv7m_realize(DeviceState *dev, Error **errp)
             return;
         }
     }
+
+    /* Tell the CPU where the NVIC is; it will fail realize if it doesn't
+     * have one.
+     */
+    s->cpu->env.nvic = &s->nvic;
+
     object_property_set_bool(OBJECT(s->cpu), true, "realized", &err);
     if (err != NULL) {
         error_propagate(errp, err);
@@ -202,7 +208,6 @@ static void armv7m_realize(DeviceState *dev, Error **errp)
     sbd = SYS_BUS_DEVICE(&s->nvic);
     sysbus_connect_irq(sbd, 0,
                        qdev_get_gpio_in(DEVICE(s->cpu), ARM_CPU_IRQ));
-    s->cpu->env.nvic = &s->nvic;
 
     memory_region_add_subregion(&s->container, 0xe000e000,
                                 sysbus_mmio_get_region(sbd, 0));

hw/intc/armv7m_nvic.c

@@ -2183,7 +2183,11 @@ static void armv7m_nvic_realize(DeviceState *dev, Error **errp)
     int regionlen;
 
     s->cpu = ARM_CPU(qemu_get_cpu(0));
-    assert(s->cpu);
+
+    if (!s->cpu || !arm_feature(&s->cpu->env, ARM_FEATURE_M)) {
+        error_setg(errp, "The NVIC can only be used with a Cortex-M CPU");
+        return;
+    }
 
     if (s->num_irq > NVIC_MAX_IRQ) {
         error_setg(errp, "num-irq %d exceeds NVIC maximum", s->num_irq);

target/arm/cpu.c

@@ -767,6 +767,24 @@ static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
         return;
     }
 
+#ifndef CONFIG_USER_ONLY
+    /* The NVIC and M-profile CPU are two halves of a single piece of
+     * hardware; trying to use one without the other is a command line
+     * error and will result in segfaults if not caught here.
+     */
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        if (!env->nvic) {
+            error_setg(errp, "This board cannot be used with Cortex-M CPUs");
+            return;
+        }
+    } else {
+        if (env->nvic) {
+            error_setg(errp, "This board can only be used with Cortex-M CPUs");
+            return;
+        }
+    }
+#endif
+
     cpu_exec_realizefn(cs, &local_err);
     if (local_err != NULL) {
         error_propagate(errp, local_err);