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

[Mark Burton]

> On 19 Jun 2015, at 09:42, Paolo Bonzini <address@hidden> wrote:
> 
> 
> 
> 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.

Yes, that is exactly correct.
Cheers
Mark.

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


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        mark.burton