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

[Philippe Mathieu-Daudé]

On 5/18/21 10:15 PM, Peter Maydell wrote:
> 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 ?

I'll defer this question to Stefan/Paolo...