versatilepb: do not run if user asks for more than 256MB RAM

The versatilepb physical address space layout only has
a 256MB region for RAM before the devices. Without a guard
on the amount of RAM requested by the user we would happily
create a RAM area that overlapped with the devices, resulting
in very confusing behaviour (typically a guest crash).

Report the problem to the user if they try to request more
RAM than the board can handle (as we do already for some
other board models).
Signed-off-by: NJean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 20161025093711.17407-1-jcd@tribudubois.net
[PMM: tidied up commit message, comments. Use error_report()
 rather than fprintf(stderr, ...).]
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/arm/versatilepb.c

@@ -198,6 +198,15 @@ static void versatile_init(MachineState *machine, int board_id)
     int done_smc = 0;
     DriveInfo *dinfo;
 
+    if (machine->ram_size > 0x10000000) {
+        /* Device starting at address 0x10000000,
+         * and memory cannot overlap with devices.
+         * Refuse to run rather than behaving very confusingly.
+         */
+        error_report("versatilepb: memory size must not exceed 256MB");
+        exit(1);
+    }
+
     if (!machine->cpu_model) {
         machine->cpu_model = "arm926";
     }