Re: [dpdk-dev] [RFC 0/2] add Tx prepare support for bonding device

[Chengchang Tang <tangchengchang@huawei.com>]

Hi
On 2021/4/20 16:33, Ananyev, Konstantin wrote:
> Hi everyone,
> 
>>
>> On 2021/4/20 9:26, Ferruh Yigit wrote:
>>> On 4/16/2021 12:04 PM, Chengchang Tang wrote:
>>>> This patch add Tx prepare for bonding device.
>>>>
>>>> Currently, the bonding driver has not implemented the callback of
>>>> rte_eth_tx_prepare function. Therefore, the TX prepare function of the
>>>> slave devices will never be invoked. When hardware offloading such as
>>>> CKSUM and TSO are enabled for some drivers, tx_prepare needs to be used
>>>> to adjust packets (for example, set correct pseudo packet headers).
>>>> Otherwise, related offloading fails and even packets are sent
>>>> incorrectly. Due to this limitation, the bonded device cannot use these
>>>> HW offloading in the Tx direction.
>>>>
>>>> Because packet sending algorithms are numerous and complex in bond PMD,
>>>> it is hard to design the callback for rte_eth_tx_prepare. In this patch,
>>>> the tx_prepare callback of bonding PMD is not implemented. Instead,
>>>> rte_eth_tx_prepare has been called in tx_burst callback. And a global
>>>> variable is introduced to control whether the bonded device need call
>>>> the rte_eth_tx_prepare. If upper-layer users need to use some TX
>>>> offloading that depend on tx_prepare , they should enable the preparation
>>>> function. In this way, the bonded device will call the rte_eth_tx_prepare
>>>> for the fast path packets in the tx_burst callback.
> 
> I admit that I didn't look at the implementation yet, but it sounds like 
> overcomplication to me. Can't we just have a new TX function for bonding PMD
> when TX offloads are enabled? And inside that function we will do:
> tx_prepare(); tx_burst(); for selected device.

The solution you mentioned is workable and may perform better. However, the current
solution is also simple and has a limited impact on performance. It is actually:
if (tx_prepare_enable)
	tx_prepare();
tx_burst();

Overall, it adds almost only one judgment to the case where the related Tx offloads
is not turned on.

> We can select this function at setup stage analysing requested by user TX offloads. 
> 

In PMDs, it is a common practice to select different Tx/Rx function during the setup
phase. But for a 'vdev' device like Bonding, we may need to think more about it.
The reasons are explained below.
> 
>>>>
>>>
>>> What do you think to add a devarg to bonding PMD to control the tx_prepare?
>>> It won't be as dynamic as API, since it can be possible to change the behavior after application is started with API, but do we really need
>> this?
>>
>> If an API is not added, unnecessary constraints may be introduced. If the
>> bonding device is created through the rte_eth_bond_create interface instead
>> devarg "vdev", this function cannot be used because devargs does not take effect
>> in this case. But from an ease-of-use perspective, adding a devarg is a good
>> idea. I will add related implementations in the later official patches.
> 
> I am also against introducing new devarg to control tx_prepare() invocation.
> I think at dev_config/queue_setup phase PMD will have enough information to decide.
> 
Currently, the community does not specify which Tx offloads need to invoke tx_prepare.
For Vdev devices such as bond, all NIC devices need to be considered. Generally,
tx_prepare is used in CKSUM and TSO. It is possible that for some NIC devices, even
CKSUM and TSO do not need to invoke tx_prepare, or for some NIC devices, there are
other Tx offloads that need to call tx_prepare. From this perspective, leaving the
choice to the user seems to be a better choice.
>>
>> If I understand correctly, the current community does not want to introduce
>> more private APIs for PMDs. However, the absence of an API on this issue would
>> introduce some unnecessary constraints, and from that point of view, I think
>> adding an API seems necessary.
>>>
>>>> Chengchang Tang (2):
>>>>    net/bonding: add Tx prepare for bonding
>>>>    app/testpmd: add cmd for bonding Tx prepare
>>>>
>>>>   app/test-pmd/cmdline.c                      | 66 +++++++++++++++++++++++++++++
>>>>   doc/guides/testpmd_app_ug/testpmd_funcs.rst |  9 ++++
>>>>   drivers/net/bonding/eth_bond_private.h      |  1 +
>>>>   drivers/net/bonding/rte_eth_bond.h          | 29 +++++++++++++
>>>>   drivers/net/bonding/rte_eth_bond_api.c      | 28 ++++++++++++
>>>>   drivers/net/bonding/rte_eth_bond_pmd.c      | 33 +++++++++++++--
>>>>   drivers/net/bonding/version.map             |  5 +++
>>>>   7 files changed, 167 insertions(+), 4 deletions(-)
>>>>
>>>> --
>>>> 2.7.4
>>>>
>>>
>>>
>>> .
>>>
>