[dpdk-dev] about rx checksum flags

[Tan, Jianfeng]

Hi Oliver,


On 5/30/2016 11:26 PM, Olivier Matz wrote:
> Hi,
>
> I'm planning to add the support for offloads in virtio-net pmd.
> It appears that the current rx flags in mbuf are not sufficient to
> describe the state of a packet received from a virtual driver.
> I think we need a way to say "the checksum in the packet data is
> not calculated, but the integrity of the data is verified".

I also met this problem :-). Glad to see you raise it up in the mail list.

>
> Currently, we have one flag for L4 (same for IP):
>
>    PKT_RX_L4_CKSUM_BAD: L4 cksum of RX pkt. is not OK.
>
> This has also another problem that has already been discussed [1]:
> if no flag is set, it is expected that the checksum is verified by
> hw, but there is no way to say "the hw does not know if the cksum
> is correct".
>
> I would like to extend this flag to a 4-state value (2 bits):
>
>   PKT_RX_L4_CKSUM_UNKNOWN: no information about the RX L4 checksum
>    -> the application should verify the checksum by sw
>
>   PKT_RX_L4_CKSUM_BAD: the L4 checksum in the packet is wrong
>    -> the application can drop the packet without additional check
>
>   PKT_RX_L4_CKSUM_GOOD: the L4 checksum in the packet is valid
>    -> the application can accept the packet without verifying the
>       checksum by sw
>
>   PKT_RX_L4_CKSUM_NONE: the L4 checksum is not correct in the packet
>   data, but the integrity of the L4 header is verified.
>    -> the application can process the packet but must not verify the
>       checksum by sw. It has to take care to recalculate the cksum
>       if the packet is transmitted (either by sw or using tx offload)
>
> To keep the compatibility with application, the old flag is kept at the
> same value, and a new flag is added. It is assumed that the behavior
> of applications was:
>
>    PKT_RX_L4_CKSUM_BAD = 0 -> packet is accepted
>    PKT_RX_L4_CKSUM_BAD = 1 -> packet is dropped
>
> The new checksum states for L4 (same for IP) would be:
>
>    old flag   new flag   meaning
>    0          0          PKT_RX_L4_CKSUM_UNKNOWN
>    1          0          PKT_RX_L4_CKSUM_BAD
>    0          1          PKT_RX_L4_CKSUM_GOOD
>    1          1          PKT_RX_L4_CKSUM_NONE
>
> With this, an old application that only checks the old flag, and
> running using a dpdk having this modification would accept GOOD and
> UNKNOWN packets (like today), drop BAD packets (like today) and
> drop NONE packets (this is a new feature that has to be explicitly
> enabled by the application).
>
>
> Any comment?

Why not take care of PKT_RX_IP_CKSUM_BAD? Is it too easy for sw to handle?

For virtio, there's only one bit, VIRTIO_NET_HDR_F_DATA_VALID, to 
indicate that checksum is valid. Shall we differentiate L3 checksum and 
L4 checksum in rte_mbuf.ol_flags?

Thanks,
Jianfeng

>
> Olivier
>
>
> [1] http://dpdk.org/ml/archives/dev/2015-January/011550.html