Re: [PATCH RFC v3 1/3] vhost: use SMP barriers instead of compiler ones.

[Ilya Maximets <i.maximets@samsung.com>]

On 18.03.2016 13:27, Xie, Huawei wrote:
> On 3/18/2016 6:23 PM, Ilya Maximets wrote:
>> On 18.03.2016 13:08, Xie, Huawei wrote:
>>> On 2/24/2016 7:47 PM, Ilya Maximets wrote:
>>>>  		 * Wait until it's our turn to add our buffer
>>>> @@ -979,7 +979,7 @@ rte_vhost_dequeue_burst(struct virtio_net *dev, uint16_t queue_id,
>>>>  		entry_success++;
>>>>  	}
>>>>  
>>>> -	rte_compiler_barrier();
>>>> +	rte_smp_rmb();
>>> smp_rmb()?
>> There is no such function 'smp_rmb' in DPDK.
>> But:
>> .../arch/arm/rte_atomic.h:#define rte_smp_rmb() rte_rmb()
>> .../arch/ppc_64/rte_atomic.h:#define rte_smp_rmb() rte_compiler_barrier()
>> .../arch/tile/rte_atomic.h:#define rte_smp_rmb() rte_compiler_barrier()
>> .../arch/x86/rte_atomic.h:#define rte_smp_rmb() rte_compiler_barrier()
> 
> I mean shoudn't be rte_smp_wmb()?

No. Here we need to be sure that copying of data from descriptor to
our local mbuf completed before 'vq->used->idx += entry_success;'.

Read memory barrier will help us with it.

In other places write barriers used because copying performed in
opposite direction.

> 
>>
>>>>  	vq->used->idx += entry_success;
>>>>  	vhost_log_used_vring(dev, vq, offsetof(struct vring_used, idx),
>>>>  			sizeof(vq->used->idx));
>>>> -- 2.5.0
>>>
>>>
> 
> 
>