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Subject: [Qemu-devel] Re: [patch uq/master 2/2] kvm-all.c: define smp_wmb
 and use it for coalesced mmio
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org, kvm@vger.kernel.org

On 02/22/2010 04:57 PM, Michael S. Tsirkin wrote:
>
>    
>>> There is no need (for this case). Older read cannot be reordered with
>>> write, writes are not reordered with other writes, writes by a single
>>> processor are observed in the same order by all processors.
>>>
>>>        
>> Well, Linux does use sfence.
>>      
> At least on 64 bit it doesnt.
>    

Right, I was looking at wmb(), not smp_wmb().

>> I imagine all arches need an instruction.  For reads as well.
>>
>> Note, gcc has a __sync_synchronize() builtin that compiles to mfence on
>> x86.  We might use that as a baseline for both rmb and wmb, and let each
>> arch override it incrementally.
>>      
> This it what my patch did. Note it only works well for recent gcc.
>    

Do you know how recent?

-- 
error compiling committee.c: too many arguments to function