q800: allow accesses to RAM area even if less memory is available

MacOS attempts a series of writes and reads over the entire RAM area in order
to determine the amount of RAM within the machine. Allow accesses to the
entire RAM area ignoring writes and always reading zero for areas where there
is no physical RAM installed to allow MacOS to detect the memory size without
faulting.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20231004083806.757242-7-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/m68k/q800.c

@@ -86,6 +86,9 @@
 
 #define MAC_CLOCK  3686418
 
+/* Size of whole RAM area */
+#define RAM_SIZE              0x40000000
+
 /*
  * Slot 0x9 is reserved for use by the in-built framebuffer whilst only
  * slots 0xc, 0xd and 0xe physically exist on the Quadra 800
@@ -218,6 +221,27 @@ static const MemoryRegionOps machine_id_ops = {
     },
 };
 
+static uint64_t ramio_read(void *opaque, hwaddr addr, unsigned size)
+{
+    return 0x0;
+}
+
+static void ramio_write(void *opaque, hwaddr addr, uint64_t val,
+                        unsigned size)
+{
+    return;
+}
+
+static const MemoryRegionOps ramio_ops = {
+    .read = ramio_read,
+    .write = ramio_write,
+    .endianness = DEVICE_BIG_ENDIAN,
+    .valid = {
+        .min_access_size = 1,
+        .max_access_size = 4,
+    },
+};
+
 static void q800_machine_init(MachineState *machine)
 {
     Q800MachineState *m = Q800_MACHINE(machine);
@@ -262,7 +286,11 @@ static void q800_machine_init(MachineState *machine)
     qemu_register_reset(main_cpu_reset, &m->cpu);
 
     /* RAM */
-    memory_region_add_subregion(get_system_memory(), 0, machine->ram);
+    memory_region_init_io(&m->ramio, OBJECT(machine), &ramio_ops, &m->ramio,
+                          "ram", RAM_SIZE);
+    memory_region_add_subregion(get_system_memory(), 0x0, &m->ramio);
+
+    memory_region_add_subregion(&m->ramio, 0, machine->ram);
 
     /*
      * Create container for all IO devices

include/hw/m68k/q800.h

@@ -60,6 +60,7 @@ struct Q800MachineState {
     MacfbNubusState macfb;
     DJMEMCState djmemc;
     IOSBState iosb;
+    MemoryRegion ramio;
     MemoryRegion macio;
     MemoryRegion macio_alias;
     MemoryRegion machine_id;