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Date: Mon, 22 Feb 2010 16:57:29 +0200
From: Avi Kivity <avi@redhat.com>
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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: Marcelo Tosatti <mtosatti@redhat.com>
Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>

On 02/22/2010 04:45 PM, Marcelo Tosatti wrote:
> On Mon, Feb 22, 2010 at 04:23:32PM +0200, Avi Kivity wrote:
>    
>> On 02/22/2010 03:59 PM, Marcelo Tosatti wrote:
>>      
>>> Cc: "Michael S. Tsirkin"<mst@redhat.com>
>>> Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
>>>
>>> Index: qemu/kvm-all.c
>>> ===================================================================
>>> --- qemu.orig/kvm-all.c
>>> +++ qemu/kvm-all.c
>>> @@ -718,6 +718,9 @@ static int kvm_handle_io(uint16_t port,
>>>       return 1;
>>>   }
>>>
>>> +/* FIXME: arch dependant, x86 version */
>>> +#define smp_wmb()   asm volatile("" ::: "memory")
>>> +
>>>        
>> sfence?
>>      
> 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.  Perhaps it's only needed for WC writes 
(movnti and friends), but better be careful here.

>> what about other arches?
>>      
> They need to be fixed? PPC needs an instruction apparently.
>
> Is there any objection to including this patch?
>    

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.

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