Re: [Qemu-devel] [RFC 01/30] softmmu: add cmpxchg helpers

[Richard Henderson]

On 06/27/2016 02:19 PM, Emilio G. Cota wrote:
> > Host endian operation?
>
> I forgot to add byte ordering in the cover letter under "why this is
> an RFC" -- I admit I'm confused by all the macro trickery done for
> regular loads and stores.
>
> We store data in memory as per the guests' byte ordering, right?

Sometimes.  The guest can also explicitly request a reversed byte load/store. 
This is used both for explicit byte-reversing instructions (e.g. x86 movbe) and 
toggling the system byte order (arm setbe).


> If so, I don't see how it would be possible to leverage the
> host compiler for things like atomic_add -- we'd increment garbage,
> not a meaningful value.

All you need to do is byte-reverse the data.

  bswap(a + b) == bswap(a) + bswap(b).


r~