Re: [RFC PATCH 14/25] qemu/bswap: Introduce load/store for aligned pointer

[Peter Maydell]

On Tue, 18 May 2021 at 19:38, Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>
> When the pointer alignment is known to be safe, we can
> directly swap the data in place, without having to rely
> on the compiler builtin code.
>
> Load/store methods expecting aligned pointer use the 'a'
> infix. For example to read a 16-bit unsigned value stored
> in little endianess at an unaligned pointer:
>
>   val = lduw_le_p(&unaligned_ptr);
>
> then to store it in big endianess at an aligned pointer:
>
>   stw_be_ap(&aligned_ptr, val);

It sounded from the bug report as if the desired effect
was "this access is atomic". Nothing in the documentation here
makes that guarantee of the implementation -- it merely imposes
an extra requirement on the caller that the pointer alignment
is "safe" (which term it does not define...) and a valid
implementation would be to implement the "aligned" versions
identically to the "unaligned" versions...

Q: should the functions at the bottom of this stack of APIs
be using something from the atomic.h header? If not, why not?
Do we need any of the other atomic primitives ?

thanks
-- PMM