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Date: Mon, 24 May 2010 09:37:05 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, Rusty Russell <rusty@rustcorp.com.au>, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org

On 05/23/2010 07:30 PM, Michael S. Tsirkin wrote:
>
>    
>>> Maybe we should use atomics on index then?
>>>
>>>        
>> This should only be helpful if you access the cacheline several times in
>> a row.  That's not the case in virtio (or here).
>>      
> So why does it help?
>    

We actually do access the cacheline several times in a row here (but not 
in virtio?):

> 		case SHARE:
> 			while (count<  MAX_BOUNCES) {
> 				/* Spin waiting for other side to change it. */
> 				while (counter->cacheline1 != count);
>    

Broadcast a read request.

> 				count++;
> 				counter->cacheline1 = count;
>    

Broadcast an invalidate request.

> 				count++;
> 			}
> 			break;
>
> 		case LOCKSHARE:
> 			while (count<  MAX_BOUNCES) {
> 				/* Spin waiting for other side to change it. */
> 				while (__sync_val_compare_and_swap(&counter->cacheline1, count, count+1)
> 				       != count);
>    

Broadcast a 'read for ownership' request.

> 				count += 2;
> 			}
> 			break;
>    

So RMW should certainly by faster using single-instruction RMW 
operations (or using prefetchw).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.