Re: [Qemu-devel] [RFC PATCH V3] Use atomic cmpxchg to atomically check the exclusive value in a STREX

[Paolo Bonzini]

On 19/06/2015 09:40, Mark Burton wrote:
>> On 19/06/2015 09:29, Mark Burton wrote:
>>> Does anybody know if the current atomic_cmpxchg will support
>>> 64 bit on a (normal) 32 bit x86, or do we need to special
>>> case that with cmpxchg8b ? (I get the impression that it will
>>> automatically use cmpxchg8b, but not cmpxchg16b - but I’m by
>>> no means sure).
>> 
>> Both cmpxchg8b and cmpxchg16b are used, respectively on 32-bit
>> and 64-bit x86.
> 
> Thanks Paolo, so  we are OK for x86, but we would need to disable
> multi-thread for other 32 bit hosts, and provide a correct
> implementation for non multi-thread…

But Alvise's implementation for example would work there.  It is just
this optimization (that is also not architecturally correct on ARM) that
is problematic.

Paolo

> You dont happen to know of a
> convenient macro we can use to test for ’32 bit hosts that dont
> support 64bit cmpxchg ….’