hw/arm_boot.c: Check for RAM sizes exceeding ATAGS capacity

The legacy ATAGS format for passing information to the kernel only
allows RAM sizes which fit in 32 bits; enforce this restriction
rather than silently doing something weird.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>

hw/arm_boot.c

@@ -399,6 +399,12 @@ void arm_load_kernel(ARMCPU *cpu, struct arm_boot_info *info)
             bootloader[5] = dtb_start;
         } else {
             bootloader[5] = info->loader_start + KERNEL_ARGS_ADDR;
+            if (info->ram_size >= (1ULL << 32)) {
+                fprintf(stderr, "qemu: RAM size must be less than 4GB to boot"
+                        " Linux kernel using ATAGS (try passing a device tree"
+                        " using -dtb)\n");
+                exit(1);
+            }
         }
         bootloader[6] = entry;
         for (n = 0; n < sizeof(bootloader) / 4; n++) {