Re: [PATCH] [PATCH] system/memory: Fix max access size

[Peter Maydell]

On Mon, 22 Jul 2024 at 17:51, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On Mon, Jul 22, 2024 at 4:29 PM Philippe Mathieu-Daudé
> <philmd@linaro.org> wrote:
> > It seems this model could be simplified using .min_access_size = 8,
> > letting the core memory layer handle the % 4 byte accesses (the
> > read path is obvious, the write one a bit less). Nothing urgent,
> > possibly a BitSizedTask.
>
> The memory access core is not able to do a read-modify-write
> operation. You can get a smaller read from a bigger one or split a
> large read/write into many small ones, but it cannot convert a small
> write into a large one. Even splitting a large read/write into many
> small ones is only possible if they don't have side effects.

IIRC it will convert a small write into a large one, but
it does so by zero-extending the value and doing a single
large write. (This is the behaviour you want for some
devices.)

thanks
-- PMM