Re: about rx checksum flags

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

On Tue, May 31, 2016 at 10:43:29AM +0800, Tan, Jianfeng wrote:
> 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?

I thought PKT_RX_IP_CKSUM_BAD was to be modified in a similar fashion, but
since you raise the issue, mlx4/mlx5 need this as well. These boards only
report "good" checksums for L3 and L4.

> 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
> 

-- 
Adrien Mazarguil
6WIND