Re: [PATCH 13/30] q800: allow accesses to RAM area even if less memory is available

[Philippe Mathieu-Daudé]

On 24/5/23 23:10, Mark Cave-Ayland wrote:
> 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>
> ---
>   hw/m68k/q800.c         | 30 +++++++++++++++++++++++++++++-
>   include/hw/m68k/q800.h |  1 +
>   2 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/hw/m68k/q800.c b/hw/m68k/q800.c
> index 8310670ec2..d12aeaa20e 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
> @@ -452,6 +455,27 @@ static const MemoryRegionOps machine_id_ops = {
>       },
>   };
>   
> +static uint64_t ramio_read(void *opaque, hwaddr addr, unsigned size)
> +{

Similar to empty_slot_init().

Personally I like to follow firmware detection using trace events here.

Maybe s/ramio/ram/.

> +    return 0x0;
> +}


>   static void q800_machine_init(MachineState *machine)
>   {
>       Q800MachineState *m = Q800_MACHINE(machine);
> @@ -497,7 +521,11 @@ 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, NULL, &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);

Yes!

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>