hw/arm/boot: Report error if there is no fw_cfg device in the machine

If the user provides both a BIOS/firmware image and also a guest
kernel filename, arm_setup_firmware_boot() will pass the
kernel image to the firmware via the fw_cfg device. However we
weren't checking whether there really was a fw_cfg device present,
and if there wasn't we would crash.

This crash can be provoked with a command line such as
 qemu-system-aarch64 -M raspi3 -kernel /dev/null -bios /dev/null -display none

It is currently only possible on the raspi3 machine, because unless
the machine sets info->firmware_loaded we won't call
arm_setup_firmware_boot(), and the only machines which set that are:
 * virt (has a fw-cfg device)
 * sbsa-ref (checks itself for kernel_filename && firmware_loaded)
 * raspi3 (crashes)

But this is an unfortunate beartrap to leave for future machine
model implementors, so we should handle this situation in boot.c.

Check in arm_setup_firmware_boot() whether the fw-cfg device exists
before trying to load files into it, and if it doesn't exist then
exit with a hopefully helpful error message.

Because we now handle this check in a machine-agnostic way, we
can remove the check from sbsa-ref.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/503
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210726163351.32086-1-peter.maydell@linaro.org

hw/arm/boot.c

@@ -1243,6 +1243,15 @@ static void arm_setup_firmware_boot(ARMCPU *cpu, struct arm_boot_info *info)
         bool try_decompressing_kernel;
 
         fw_cfg = fw_cfg_find();
+
+        if (!fw_cfg) {
+            error_report("This machine type does not support loading both "
+                         "a guest firmware/BIOS image and a guest kernel at "
+                         "the same time. You should change your QEMU command "
+                         "line to specify one or the other, but not both.");
+            exit(1);
+        }
+
         try_decompressing_kernel = arm_feature(&cpu->env,
                                                ARM_FEATURE_AARCH64);
 

hw/arm/sbsa-ref.c

@@ -691,13 +691,6 @@ static void sbsa_ref_init(MachineState *machine)
 
     firmware_loaded = sbsa_firmware_init(sms, sysmem, secure_sysmem);
 
-    if (machine->kernel_filename && firmware_loaded) {
-        error_report("sbsa-ref: No fw_cfg device on this machine, "
-                     "so -kernel option is not supported when firmware loaded, "
-                     "please load OS from hard disk instead");
-        exit(1);
-    }
-
     /*
      * This machine has EL3 enabled, external firmware should supply PSCI
      * implementation, so the QEMU's internal PSCI is disabled.