[dpdk-dev] about rx checksum flags

[Olivier Matz <olivier.matz@6wind.com>]

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".

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?

Olivier


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