Re: [Qemu-devel] [RFC PATCH] target-arm: protect cpu_exclusive_*.

[Paolo Bonzini]

On 18/12/2014 16:05, Alexander Graf wrote:
> Yeah, the semantics should be tied to what TM would give you. We can
> always be more safe than TM in our fallback implementation, but I
> wouldn't want to see semantic optimizations tied to the MMIO
> implementation put in.
> 
> This is mostly theory though, try to write the code and see where things
> fall apart, then we'll be in a much better position to rationalize on
> where to do things differently.

Yeah, this is why I think LL/SC ops for TCG are more interesting and
important than CMPXCHG.  Also because x86 doesn't have just CMPXCHG, it
also has XADD which you'd have to implement anyway as a LL/SC or CMPXCHG
loop, so CMPXCHG doesn't get you all the way.

Paolo