Re: [dpdk-dev] [PATCH v8 10/10] app/testpmd:test VxLAN Tx checksum offload

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

Hello Jijiang,

On 10/27/2014 03:13 AM, Jijiang Liu wrote:
> Add test cases in testpmd to test VxLAN Tx Checksum offload, which include
>   - IPv4 and IPv6 packet
>   - outer L3, inner L3 and L4 checksum offload for Tx side.
>
> Signed-off-by: Jijiang Liu <jijiang.liu@intel.com>

I'm trying to port the test of TSO in csum_only.c which was originally
part of this patch [1].

Meanwhile, the source was modified by the patch provided by the email
I'm replying to (also available at [2]), and I would like to understand
what is the purpose of it.

First, the code checks if the mbuf has the flag PKT_RX_TUNNEL_IPV4_HDR.
What is the meaning of this flag? It was added by [3], but there is no
description in comments or in the commit log explaining in which case
this flag is set by the driver. The name supposes that this flag is
set when the received packet is an IPv4 tunnel, but the commit log talks
about vxlan.

Then, if this flag was present, the code assumes it's a vxlan packet.
If one of inner checksum is specified by the user in cmdline, the flag
PKT_TX_VXLAN_CKSUM is added to the mbuf. This flag definition was added
by [4] (at the wrong place, I'll fix it in my patchset). What is the
meaning of this flag? Is it enough to checksum outer L3, inner L3, and
inner L4 as specified in commit log? If yes, why are the other flags
PKT_TX_IPV4_CSUM, PKT_TX_UDP_CKSUM, (...) added in the mbuf later?
In my comprehension, these flags are needed in addition to
PKT_TX_VXLAN_CKSUM to do the checksum of the inner headers.

In general, I need to understand how to use all the new offloading
stuff. In [5], new fields were added in the mbuf structure
(inner_l3_len and inner_l2_len). I'm not sure I understand which fields
have to be filled. Below is my understanding, can you please check that
it is correct?

A- To send a Ether/IP/TCP packet and process IP and TCP TX
    checksum in the NIC:

   - set l2_len = 14, l3_len = 20,
     ol_flags = PKT_TX_IP_CKSUM | PKT_TX_TCP_CKSUM,
     write all checksums to 0 in the packet

B- To send a Ether/IP/UDP/vxlan/Ether/IP/TCP packet and
    process inner IP and inner TCP TX checksum in the NIC:

   - set l2_len = 14+20+8+8+14, l3_len = 20,
     ol_flags = PKT_TX_IP_CKSUM | PKT_TX_TCP_CKSUM,
     write all checksums to 0 in the packet

C- To send a Ether/IP/UDP/vxlan/Ether/IP/TCP packet and
    process outer IP and UDP, plus inner IP and inner TCP TX
    checksum in the NIC:

   - set l2_len = 14, l3_len = 20, inner_l2_len = 14,
     inner_l3_len = 20,
     ol_flags = PKT_TX_IP_CKSUM | PKT_TX_TCP_CKSUM | PKT_TX_VXLAN_CKSUM,
     write all checksums to 0 in the packet

D- To send a Ether/IP/UDP/vxlan/Ether/IP/TCP packet and
    process outer IP and UDP TX checksum in the NIC:

   - set l2_len = 14, l3_len = 20,
     ol_flags = PKT_TX_IP_CKSUM | PKT_TX_UDP_CKSUM,
     write all checksums to 0 in the packet

First, can you confirm this? I think it is very important to
document it, as this is a public API that can be used by DPDK users.

To validate my modifications, I will try to reproduce the test plan
described in [6]. The test report contains useful information but
I'm not sure to understand the following:

    Enable outer IP,UDP,TCP,SCTP and inner IP,UDP checksum offload
    when inner L4 protocal is UDP.
       testpmd>tx_checksum set 0 0xf3
    Enable outer IP,UDP,TCP,SCTP and inner IP,TCP,SCTP checksum offload
    when inner L4 protocal is TCP or SCTP.
       testpmd>tx_checksum set 0 0xfd

Can you give details about the signification of the bits used in the
tx_checksum command? For instance, there is only one flag to enable
tx checksum in the mbuf, so I don't understand how it's possible to
do 0x44 = (inner tcp + outer tcp).

Today, there are hard-coded values for flags: 4 bits (0-3) for legacy
checksums, 4 bits for inner checksums (4-7), and one bit (bit 11?!)
for vlan header. These bits are checked without defines at several
places in the code. Moreover, there are some places in code where
the testpmd port flags and the mbuf flags are mixed up. Example in
macswap.c : mb->ol_flags = txp->tx_ol_flags;

I suggest to remove all hardcoded values except in
cmd_tx_cksum_set_parsed() and replace them by the flags definitions
from rte_mbuf.h. As a result, the new possible arguments of
cmd_tx_cksum_set_parsed() will map the mbuf flags:
   - a flag to enable ip tx cksum
   - a flag to enable udp tx cksum
   - a flag to enable tcp tx cksum
   - a flag to enable sctp tx cksum
   - a flag to enable vxlan tx cksum
If vxlan tx cksum is set, therefore the other considered checksum
will concern the inner headers. Do you think it matches your needs?

I think that the csum_only forward engine is now a bit complicated.
As it's an example that shows how to configure the tx checksum, I
think its behavior should be described somewhere, maybe in a comment in 
the file.

By the way, I was looking at the mbuf structure, and I see that a new
packet_type field was added. There is no explanation about what this
field should contain and how we shall use it (in commit log or in
comments). Can you please explain it?

To conclude, I'd like to propose a merge of the TSO series but
I'm currently blocked by these questions. Please, could you
help me to progress on this?

Regards,
Olivier


[1] http://dpdk.org/ml/archives/dev/2014-May/002549.html
[2] http://dpdk.org/ml/archives/dev/2014-October/007156.html
[3] 
http://dpdk.org/browse/dpdk/commit/lib/librte_mbuf/rte_mbuf.h?id=15dbb63ef9e9f108e7dcd837b88234f27a1ec258
[4] 
http://dpdk.org/browse/dpdk/commit/lib/librte_mbuf/rte_mbuf.h?id=77b8301733c3cd946648ca4a1375e3db0a952216
[5] 
http://dpdk.org/browse/dpdk/commit/lib/librte_mbuf/rte_mbuf.h?id=73b7d59cf4f6faf5ccd83ce706473e75c6fb8c3b
[6] http://dpdk.org/ml/archives/dev/2014-October/007157.html