Re: [dpdk-dev] [PATCH] net/tap: Allow all-zero checksum for UDP over IPv4

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 11/10/2020 4:01 PM, Morten Brørup wrote:
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit
>> Sent: Tuesday, November 10, 2020 3:47 PM
>>
>> On 11/9/2020 2:22 PM, Michael Pfeiffer wrote:
>>> Unlike TCP, UDP checksums are optional and may be zero to indicate "not
>>> set" [RFC 768] (except for IPv6, where this prohibited [RFC 8200]). Add
>>> this special case to the checksum offload emulation in net/tap.
>>>
>>> Signed-off-by: Michael Pfeiffer <michael.pfeiffer@tu-ilmenau.de>
>>> ---
>>>    drivers/net/tap/rte_eth_tap.c | 13 +++++++++++--
>>>    1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/tap/rte_eth_tap.c
>> b/drivers/net/tap/rte_eth_tap.c
>>> index 2f8abb12c..e486b41c5 100644
>>> --- a/drivers/net/tap/rte_eth_tap.c
>>> +++ b/drivers/net/tap/rte_eth_tap.c
>>> @@ -303,6 +303,7 @@ tap_verify_csum(struct rte_mbuf *mbuf)
>>>    	uint16_t cksum = 0;
>>>    	void *l3_hdr;
>>>    	void *l4_hdr;
>>> +	struct rte_udp_hdr *udp_hdr;
>>>
>>>    	if (l2 == RTE_PTYPE_L2_ETHER_VLAN)
>>>    		l2_len += 4;
>>> @@ -349,10 +350,18 @@ tap_verify_csum(struct rte_mbuf *mbuf)
>>>    		/* Don't verify checksum for multi-segment packets. */
>>>    		if (mbuf->nb_segs > 1)
>>>    			return;
>>> -		if (l3 == RTE_PTYPE_L3_IPV4)
>>> +		if (l3 == RTE_PTYPE_L3_IPV4) {
>>> +			if (l4 == RTE_PTYPE_L4_UDP) {
>>> +				udp_hdr = (struct rte_udp_hdr *)l4_hdr;
>>> +				if (udp_hdr->dgram_cksum == 0) {
>>> +					mbuf->ol_flags |= PKT_RX_L4_CKSUM_NONE;
>>> +					return;
>>> +				}
>>> +			}
>>>    			cksum = ~rte_ipv4_udptcp_cksum(l3_hdr, l4_hdr);
>>> -		else if (l3 == RTE_PTYPE_L3_IPV6)
>>> +		} else if (l3 == RTE_PTYPE_L3_IPV6) {
>>>    			cksum = ~rte_ipv6_udptcp_cksum(l3_hdr, l4_hdr);
>>> +		}
>>>    		mbuf->ol_flags |= cksum ?
>>>    			PKT_RX_L4_CKSUM_BAD :
>>>    			PKT_RX_L4_CKSUM_GOOD;
>>>
>>
>> While checking this I stuck with following part:
>>
>>    cksum = ~rte_ipv6_udptcp_cksum(l3_hdr, l4_hdr);
>>    ...
>>     mbuf->ol_flags |= cksum ?
>>     	PKT_RX_L4_CKSUM_BAD :
>>    	PKT_RX_L4_CKSUM_GOOD;
>>
>>
>> Is this correct, or am I missing something, can intention be '!' here
>> instead of
>> '~' ?
> 
> It is correct. The packet's checksum is calculated by rte_ipv6_udptcp_cksum(), and it should be 0xFFFF. The '~' operation makes cksum 0 iff the calculated checksum is 0xFFFF.
> 

Yep, figure that out late,
as far as I understand when the checksum value is zero, 
'rte_ipv6_udptcp_cksum()' will return the checksum value and when checksum is 
correct in the packet, function will return 0xFFFF, this is based on checksum 
calculation, is this right?