Subject: Re: [PATCH 4/5] ibmveth: Add support for Large Receive Offload

From: Thomas Falcon <[email-masked]>
Date: Tue, 21 Apr 2015 13:59:30 -0500

On 04/14/2015 05:00 PM, Eric Dumazet wrote:
> On Tue, 2015-04-14 at 15:35 -0500, Thomas Falcon wrote:
>> Enables receiving large packets from other LPARs. These packets
>> have a -1 IP header checksum, so we must recalculate to have
>> a valid checksum.
>>
>> Signed-off-by: Brian King <[email-masked]>
>> Signed-off-by: Thomas Falcon <[email-masked]>
>> ---
>>  drivers/net/ethernet/ibm/ibmveth.c | 15 ++++++++++++++-
>>  1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
>> index 08970c7..05eaca6a 100644
>> --- a/drivers/net/ethernet/ibm/ibmveth.c
>> +++ b/drivers/net/ethernet/ibm/ibmveth.c
>> @@ -1092,6 +1092,7 @@ static int ibmveth_poll(struct napi_struct *napi, int budget)
>>  	struct net_device *netdev = adapter->netdev;
>>  	int frames_processed = 0;
>>  	unsigned long lpar_rc;
>> +	struct iphdr *iph;
>>  
>>  restart_poll:
>>  	while (frames_processed < budget) {
>> @@ -1134,8 +1135,20 @@ restart_poll:
>>  			skb_put(skb, length);
>>  			skb->protocol = eth_type_trans(skb, netdev);
>>  
>> -			if (csum_good)
>> +			if (csum_good) {
>>  				skb->ip_summed = CHECKSUM_UNNECESSARY;
>> +				if (be16_to_cpu(skb->protocol) == ETH_P_IP) {
>> +					skb_set_network_header(skb, 0);
>> +					skb_set_transport_header(skb, sizeof(struct iphdr));
>> +					iph = ip_hdr(skb);
>> +
>> +					/* If the IP checksum is not offloaded and if the packet
>> +					 *  is large send, the checksum must be rebuilt.
>> +					 */
>> +					if (iph->check == 0xffff)
>> +						iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
>
> How can this possibly work ?
>
> Normally you would have to set iph->check to 0 before calling
> ip_fast_csum(), as done in ip_send_check()
I don't have an answer for why I weren't seeing any problems while testing this, but I'll go back and set iph->check to zero and retest.  Thanks for noticing this.
>
>
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