From mboxrd@z Thu Jan  1 00:00:00 1970
From: Maxime Coquelin <maxime.coquelin@redhat.com>
Subject: Re: [PATCH v2] vhost: Only access header if offloading
 is supported in dequeue path
Date: Fri, 14 Oct 2016 09:24:16 +0200
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 olivier.matz@6wind.com, stephen@networkplumber.org
To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
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On 10/11/2016 11:01 AM, Yuanhan Liu wrote:
> On Tue, Oct 11, 2016 at 09:45:27AM +0200, Maxime Coquelin wrote:
>> @@ -684,12 +699,12 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>>  		  struct rte_mempool *mbuf_pool)
>>  {
>>  	struct vring_desc *desc;
>> -	uint64_t desc_addr;
>> +	uint64_t desc_addr = 0;
>>  	uint32_t desc_avail, desc_offset;
>>  	uint32_t mbuf_avail, mbuf_offset;
>>  	uint32_t cpy_len;
>>  	struct rte_mbuf *cur = m, *prev = m;
>> -	struct virtio_net_hdr *hdr;
>> +	struct virtio_net_hdr *hdr = NULL;
>>  	/* A counter to avoid desc dead loop chain */
>>  	uint32_t nr_desc = 1;
>>
>> @@ -698,12 +713,14 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>>  			(desc->flags & VRING_DESC_F_INDIRECT))
>>  		return -1;
>>
>> -	desc_addr = gpa_to_vva(dev, desc->addr);
>> -	if (unlikely(!desc_addr))
>> -		return -1;
>> +	if (virtio_net_with_host_offload(dev)) {
>> +		desc_addr = gpa_to_vva(dev, desc->addr);
>> +		if (unlikely(!desc_addr))
>> +			return -1;
>>
>> -	hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
>> -	rte_prefetch0(hdr);
>> +		hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
>> +		rte_prefetch0(hdr);
>> +	}
>>
>>  	/*
>>  	 * A virtio driver normally uses at least 2 desc buffers
>> @@ -720,18 +737,24 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>>  		if (unlikely(!desc_addr))
>>  			return -1;
>>
>> -		rte_prefetch0((void *)(uintptr_t)desc_addr);
>> -
>>  		desc_offset = 0;
>>  		desc_avail  = desc->len;
>>  		nr_desc    += 1;
>> -
>> -		PRINT_PACKET(dev, (uintptr_t)desc_addr, desc->len, 0);
>>  	} else {
>> +		if (!desc_addr) {
>> +			desc_addr = gpa_to_vva(dev, desc->addr);
>> +			if (unlikely(!desc_addr))
>> +				return -1;
>> +		}
>> +
>
> I think this piece of code make things a bit complex. I think what you
> want to achieve is, besides saving hdr prefetch, to save one call to
> gpa_to_vva() for the non-ANY_LAYOUT case. Does that matter too much?
>
> How about just saving the hdr prefetch?
>
> 	if (virtio_net_with_host_offload(dev)) {
> 		hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
> 		rte_prefetch0(hdr);
> 	}
Oops, you reply slipped through the cracks...

You're right, it doesn't matter too much, the thing to avoid id
definitely the hdr prefetch and access.

I'm sending a v3 now.

Thanks,
Maxime