Re: [PATCH 5/6] exec: Restrict 32-bit CPUs to 32-bit address space

[Peter Maydell]

On Sun, 31 May 2020 at 18:54, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>
> It is pointless to have 32-bit CPUs see a 64-bit address
> space, when they can only address the 32 lower bits.
>
> Only create CPU address space with a size it can address.
> This makes HMP 'info mtree' command easier to understand
> (on 32-bit CPUs).

> diff --git a/exec.c b/exec.c
> index 5162f0d12f..d6809a9447 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2962,9 +2962,17 @@ static void tcg_commit(MemoryListener *listener)
>
>  static void memory_map_init(void)
>  {
> +    uint64_t system_memory_size;
> +
> +#if TARGET_LONG_BITS >= 64
> +    system_memory_size = UINT64_MAX;
> +#else
> +    system_memory_size = 1ULL << TARGET_LONG_BITS;
> +#endif

TARGET_LONG_BITS is a description of the CPU's virtual
address size; but the size of the system_memory memory
region is related to the CPU's physical address size[*].
In particular, for the Arm Cortex-A15 (and any other
32-bit CPU with LPAE) TARGET_LONG_BITS is 32 but the CPU
can address more than 32 bits of physical memory.

[*] Strictly speaking, it would depend on the
maximum physical address size used by any transaction
master in the system -- in theory you could have a
32-bit-only CPU and a DMA controller that could be
programmed with 64-bit addresses. In practice the
CPU can generally address at least as much of the
physical address space as any other transaction master.

thanks
-- PMM