[PULL 06/20] q800: allow accesses to RAM area even if less memory is available

[Laurent Vivier <laurent@vivier.eu>]

From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

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>
---
 include/hw/m68k/q800.h |  1 +
 hw/m68k/q800.c         | 30 +++++++++++++++++++++++++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/include/hw/m68k/q800.h b/include/hw/m68k/q800.h
index 98097165d954..04e4e0bce35a 100644
--- a/include/hw/m68k/q800.h
+++ b/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;
diff --git a/hw/m68k/q800.c b/hw/m68k/q800.c
index 081b95e9cf66..3209309173f3 100644
--- a/hw/m68k/q800.c
+++ b/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
-- 
2.41.0