Re: [dpdk-dev] about rx checksum flags

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Mon, May 30, 2016 at 05:26:21PM +0200, Olivier Matz wrote:
> Hi,
> 
> I'm planning to add the support for offloads in virtio-net pmd.

Good to know, and thanks!

> 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

This is good to have, which could save some burderns of cksum
validation when using kernel virtio-net (that has a TCP/IP stack
on top of it) and vhost-user combo.

> 
>  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)

I like the explanation you made at [1] better :)

So in general, I think this proposal is good to have.

	--yliu

> 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