[dpdk-dev] [PATCH v4] vhost: use SMP barriers instead of compiler ones.

[Xie, Huawei]

On 3/21/2016 10:07 PM, Ananyev, Konstantin wrote:
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ilya Maximets
>> Sent: Monday, March 21, 2016 4:50 AM
>> To: Yuanhan Liu
>> Cc: dev at dpdk.org; Xie, Huawei; Dyasly Sergey
>> Subject: Re: [dpdk-dev] [PATCH v4] vhost: use SMP barriers instead of compiler ones.
>>
>>
>>
>> On 18.03.2016 15:41, Yuanhan Liu wrote:
>>> On Fri, Mar 18, 2016 at 03:23:53PM +0300, Ilya Maximets wrote:
>>>> Since commit 4c02e453cc62 ("eal: introduce SMP memory barriers") virtio
>>>> uses architecture dependent SMP barriers. vHost should use them too.
>>>>
>>>> Fixes: 4c02e453cc62 ("eal: introduce SMP memory barriers")
>>>>
>>>> Signed-off-by: Ilya Maximets <i.maximets at samsung.com>
>>>> ---
>>>>  lib/librte_vhost/vhost_rxtx.c | 7 ++++---
>>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
>>>> index b4da665..859c669 100644
>>>> --- a/lib/librte_vhost/vhost_rxtx.c
>>>> +++ b/lib/librte_vhost/vhost_rxtx.c
>>>> @@ -315,7 +315,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
>>>>  			rte_prefetch0(&vq->desc[desc_indexes[i+1]]);
>>>>  	}
>>>>
>>>> -	rte_compiler_barrier();
>>>> +	rte_smp_wmb();
>>>>
>>>>  	/* Wait until it's our turn to add our buffer to the used ring. */
>>>>  	while (unlikely(vq->last_used_idx != res_start_idx))
>>>> @@ -565,7 +565,7 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t queue_id,
>>>>
>>>>  		nr_used = copy_mbuf_to_desc_mergeable(dev, vq, start, end,
>>>>  						      pkts[pkt_idx]);
>>>> -		rte_compiler_barrier();
>>>> +		rte_smp_wmb();
>>>>
>>>>  		/*
>>>>  		 * Wait until it's our turn to add our buffer
>>>> @@ -923,7 +923,8 @@ rte_vhost_dequeue_burst(struct virtio_net *dev, uint16_t queue_id,
>>>>  				sizeof(vq->used->ring[used_idx]));
>>>>  	}
>>>>
>>>> -	rte_compiler_barrier();
>>>> +	rte_smp_wmb();
>>>> +	rte_smp_rmb();
>>> rte_smp_mb?
>> rte_smp_mb() is a real mm_fence() on x86. And we don't need to synchronize reads with
>> writes here, only reads with reads and writes with writes. It is enough because next
>> increment uses read and write. Pair of barriers is better because it will not impact
>> on performance on x86.
> Not arguing about that particular patch, just a question:
> Why do we have:
> #define rte_smp_mb() rte_mb()

Konstantine, actually smp_mb is defined as mfence while smp_r/wmb are
defined as barrier in kernel_src/arch/x86/include/asm/barrier.h.

> for  x86?
> Why not just:
> #define rte_smp_mb() rte_compiler_barrier()
> here?
> I meant for situations when we do need real mfence, there is an 'rte_mb' to use.
> Konstantin
>
>> Best regards, Ilya Maximets.