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

[Mark Burton]

for the 1<<size thing - I think that code has been used elsewhere, which is a 
little worrying - I’ll check.

> On 18 Jun 2015, at 17:56, Peter Maydell <address@hidden> wrote:
> 
> On 18 June 2015 at 16:44,  <address@hidden> wrote:
>> +        uint64_t oldval, *p;
>> +        p = address_space_map(cs->as, paddr, &len, true);
>> +        if (len == 8 << size) {
>> +            oldval = (uint64_t)env->exclusive_val;
>> +            result = (atomic_cmpxchg(p, oldval, (uint64_t)newval) == 
>> oldval);
> 
> You can't do an atomic operation on a type that's larger than
> the pointer size of the host system. That means that for
> code that isn't host specific, like this, in practice you
> can't do an atomic operation on a larger size than 4 bytes.
> 

I thought they were polymorphic across all types, I didn’t notice the caveat of 
the size, sorry about that. That makes things more entertaining :-)

Cheers
Mark.



> (It happens to work on x86, I think, but this won't build
> on ppc32, for instance.)
> 
> thanks
> -- PMM


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        mark.burton