[Qemu-devel] Re: [patch uq/master 2/2] kvm-all.c: define smp_wmb and use it for coalesced mmio

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Feb 22, 2010 at 05:08:00PM +0200, Avi Kivity wrote:
> 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?

IIRC 4.3 has broken implementation.
4.4 seems OK as far as I can tell.

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