Subject: Re: [PATCH net-next RFC v1 07/10] nvme-tcp : Recalculate crc in the end of the capsule

From: Boris Pismenny <[email-masked]>
Date: Sun, 8 Nov 2020 16:46:45 +0200

On 09/10/2020 1:44, Sagi Grimberg wrote:
>> crc offload of the nvme capsule. Check if all the skb bits
>> are on, and if not recalculate the crc in SW and check it.
> Can you clarify in the patch description that this is only
> for pdu data digest and not header digest?

Will do

>
>> This patch reworks the receive-side crc calculation to always
>> run at the end, so as to keep a single flow for both offload
>> and non-offload. This change simplifies the code, but it may degrade
>> performance for non-offload crc calculation.
> ??
>
>  From my scan it doeesn't look like you do that.. Am I missing something?
> Can you explain?

The performance of CRC data digest in the offload's fallback path may be less good compared to CRC calculation with skb_copy_and_hash.
To be clear, the fallback path is occurs when `queue->data_digest && test_bit(NVME_TCP_Q_OFF_CRC_RX, &queue->flags)`, while we receive SKBs where `skb->ddp_crc = 0`

>
>>   	rq = blk_mq_tag_to_rq(nvme_tcp_tagset(queue), pdu->command_id);
>>   	if (!rq) {
>>   		dev_err(queue->ctrl->ctrl.device,
>> @@ -992,7 +1031,7 @@ static int nvme_tcp_recv_data(struct nvme_tcp_queue *queue, struct sk_buff *skb,
>>   		recv_len = min_t(size_t, recv_len,
>>   				iov_iter_count(&req->iter));
>>   
>> -		if (queue->data_digest)
>> +		if (queue->data_digest && !test_bit(NVME_TCP_Q_OFFLOADS, &queue->flags))
>>   			ret = skb_copy_and_hash_datagram_iter(skb, *offset,
>>   				&req->iter, recv_len, queue->rcv_hash);
> This is the skb copy and hash, not clear why you say that you move this
> to the end...

See the offload fallback path below

>
>>   		else
>> @@ -1012,7 +1051,6 @@ static int nvme_tcp_recv_data(struct nvme_tcp_queue *queue, struct sk_buff *skb,
>>   
>>   	if (!queue->data_remaining) {
>>   		if (queue->data_digest) {
>> -			nvme_tcp_ddgst_final(queue->rcv_hash, &queue->exp_ddgst);
> If I instead do:
> 			if (!test_bit(NVME_TCP_Q_OFFLOADS,
>                                        &queue->flags))
> 				nvme_tcp_ddgst_final(queue->rcv_hash,
> 						     &queue->exp_ddgst);
>
> Does that help the mess in nvme_tcp_recv_ddgst?

Not really, as the code path there takes care of the fallback path, i.e. offloaded requested, but didn't succeed.

>
>>   			queue->ddgst_remaining = NVME_TCP_DIGEST_LENGTH;
>>   		} else {
>>   			if (pdu->hdr.flags & NVME_TCP_F_DATA_SUCCESS) {
>> @@ -1033,8 +1071,11 @@ static int nvme_tcp_recv_ddgst(struct nvme_tcp_queue *queue,
>>   	char *ddgst = (char *)&queue->recv_ddgst;
>>   	size_t recv_len = min_t(size_t, *len, queue->ddgst_remaining);
>>   	off_t off = NVME_TCP_DIGEST_LENGTH - queue->ddgst_remaining;
>> +	bool ddgst_offload_fail;
>>   	int ret;
>>   
>> +	if (test_bit(NVME_TCP_Q_OFFLOADS, &queue->flags))
>> +		nvme_tcp_device_ddgst_update(queue, skb);
>>   	ret = skb_copy_bits(skb, *offset, &ddgst[off], recv_len);
>>   	if (unlikely(ret))
>>   		return ret;
>> @@ -1045,12 +1086,21 @@ static int nvme_tcp_recv_ddgst(struct nvme_tcp_queue *queue,
>>   	if (queue->ddgst_remaining)
>>   		return 0;
>>   
>> -	if (queue->recv_ddgst != queue->exp_ddgst) {
>> -		dev_err(queue->ctrl->ctrl.device,
>> -			"data digest error: recv %#x expected %#x\n",
>> -			le32_to_cpu(queue->recv_ddgst),
>> -			le32_to_cpu(queue->exp_ddgst));
>> -		return -EIO;
>> +	ddgst_offload_fail = !nvme_tcp_device_ddgst_ok(queue);
>> +	if (!test_bit(NVME_TCP_Q_OFFLOADS, &queue->flags) ||
>> +	    ddgst_offload_fail) {
>> +		if (test_bit(NVME_TCP_Q_OFFLOADS, &queue->flags) &&
>> +		    ddgst_offload_fail)
>> +			nvme_tcp_crc_recalculate(queue, pdu);
>> +
>> +		nvme_tcp_ddgst_final(queue->rcv_hash, &queue->exp_ddgst);
>> +		if (queue->recv_ddgst != queue->exp_ddgst) {
>> +			dev_err(queue->ctrl->ctrl.device,
>> +				"data digest error: recv %#x expected %#x\n",
>> +				le32_to_cpu(queue->recv_ddgst),
>> +				le32_to_cpu(queue->exp_ddgst));
>> +			return -EIO;
> This gets convoluted here...

Will try to simplify, the general idea is that there are 3 paths with common code:
1. non-offload
2. offload failed
3. offload success
(1) and (2) share the code for finalizing checking the data digest, while (3) skips this entirely.

In other words, how about this:
```
          offload_fail = !nvme_tcp_ddp_ddgst_ok(queue);                                               
          offload = test_bit(NVME_TCP_Q_OFF_CRC_RX, &queue->flags);                                   
          if (!offload || offload_fail) {                                                             
                  if (offload && offload_fail) // software-fallback                                   
                          nvme_tcp_ddp_ddgst_recalc(queue, pdu);                                      
                                                                                                      
                  nvme_tcp_ddgst_final(queue->rcv_hash, &queue->exp_ddgst);                           
                  if (queue->recv_ddgst != queue->exp_ddgst) {                                        
                          dev_err(queue->ctrl->ctrl.device,                                           
                                  "data digest error: recv %#x expected %#x\n",                       
                                  le32_to_cpu(queue->recv_ddgst),                                     
                                  le32_to_cpu(queue->exp_ddgst));                                     
                          return -EIO;                                                                
                  }                                                                                   
          } 
```

>
>> +		}
>>   	}
>>   
>>   	if (pdu->hdr.flags & NVME_TCP_F_DATA_SUCCESS) {
>>