Re: [dpdk-dev] [PATCH v12 0/7] ethdev: add new Rx offload flags

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 10/18/2019 8:32 AM, Andrew Rybchenko wrote:
> Hi Ferruh,
> 
> since I've reviewed I'll reply as I understand it.
> 
> On 10/17/19 8:43 PM, Ferruh Yigit wrote:
>> On 10/17/2019 1:02 PM, pbhagavatula@marvell.com wrote:
>>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>
>>>   Add new Rx offload flags `DEV_RX_OFFLOAD_RSS_HASH` and
>>>   `DEV_RX_OFFLOAD_FLOW_MARK`. These flags can be used to
>>>   enable/disable PMD writes to rte_mbuf fields `hash.rss` and `hash.fdir.hi`
>>>   and also `ol_flags:PKT_RX_RSS` and `ol_flags:PKT_RX_FDIR`.
>> Hi Pavan,
>>
>> Initially sorry for involving late,
>>
>> When we expose an interface to the applications, they will expect those will be
>> respected by underlying PMDs.
>> As far as I can see drivers are updated to report new added Rx offload flags as
>> supported capabilities but drivers are not using those flags at all, so
>> application providing that flag won't really enable/disable anything, I think
>> this is a problem and it is wrong to lie even for the PMDs J
> 
> It is required to let applications know that the offload is supported.
> There are a number of cases when an offload cannot be disabled,
> but it does not mean that the offload must not be advertised.

Can't disable is something else, although I believe that is rare case, in this
case driver can enable/disable the RSS and representing this as an offload
capability.
But when user want to configure this offload by setting or unsetting in offload
config, driver just ignores it.

> If driver see benefits from disabling the offload (e.g. avoid delivery
> of RSS hash from NIC to host), it can do it after the patchset.

Yes but I think this patchset shouldn't ignore that disabling the feature is not
implemented yet. If those PMDs that has been updated to report the HASH
capability has RSS enabled by default, I suggest adding a check for this offload
in PMD,
if it is requested to disable (which means not requested for enable), print a
log saying disabling HASH is not supported and set this flag in the offload
configuration to say PMD is configured to calculate the HASH.
Later PMD maintainers may prefer to replace that error log with actual disable code.

> 
>> Specific to `DEV_RX_OFFLOAD_RSS_HASH`, we have already some RSS config
>> structures and it is part of the 'rte_eth_dev_configure()' API, won't it create
>> multiple way to do same thing?
> 
> No, a new offload is responsible for RSS hash delivery from NIC to host
> and fill in in mbuf returned to application on Rx.

What you have described is already happening without the new offload flag and
this is my concern that we are duplicating it.


There is a 'struct rte_eth_rxmode' (under 'struct rte_eth_conf')
which has 'enum rte_eth_rx_mq_mode mq_mode;'

If "mq_mode == ETH_MQ_RX_NONE" hash calculation is disabled, and
'mbuf::hash::rss' is not updated.

(Thanks Bruce to helping finding it out)


> 
>> And for the `ol_flags:PKT_RX_RSS` flag, it was already used to mark that
>> 'mbuf::hash::rss' is valid, right? Is there anything new related that in the set?
> 
> As I understand you mean, ol_flags::PKT_RX_RSS_HASH.
> Yes, the new offload allows say if application needs it or now.
> Basically it decouples RSS distribution and hash delivery.

Setting 'ol_flags::PKT_RX_RSS_HASH' and 'mbuf::hash::rss' already there and not
changing. I just want to clarify since this is not clear in the commit log.

Only addition is to add a new flag to control PMD to enable/disable hash
calculation (which PMDs ignore in the patch ???)

> 
>> Specific to the `DEV_RX_OFFLOAD_FLOW_MARK` and `RTE_FLOW_ACTION_FLAG`, they are
>> rte_flow actions, application can verify and later request these actions via
>> rte_flow APIs. Why we are adding an additional RX_OFFLOAD flag for them?
> 
> The reason is basically the same as above. HW needs to know in advance,
> if application is going to use flow marks and configure Rx queue to enable
> the information delivery.

What you described is done via 'rte_flow_create()' API, application will request
those actions via API and Rx queue will be configured accordingly, this is more
dynamic approach. Why application need to set this additional configuration flag?

And as above the new RX offload flags ignored by PMDs, hard to understand what
is the intention here.


Above usage of flags feels like the intention is adding some capability
information for the PMDs more that adding new offload configuration.
If so this is bigger/older problem, and instead of abusing the offload flags we
can think of an API to present device capabilities, and move features.ini
content to the API in long term.

> 
>>>   Add new packet type set function `rte_eth_dev_set_supported_ptypes`,
>>>   allows application to inform PMDs about the packet types it is interested
>>>   in. Based on ptypes requested by application PMDs can optimize the Rx path.
>> OK to the API, but why "Packet type parsing" feature updated to say it should
>> implement this API?
>> Is this API really required to say "Packet type parsing" supported by PMD?
> 
> As I understand it is not strictly required, but related to the feature.

I am OK with "related", but it is documented as "implements", so doc says it is
required.

> 
>>>   For example, if a given PMD doesn't support any packet types that the
>>>   application is interested in then the application can disable[1] writes to
>>>   `mbuf.packet_type` done by the PMD and use a software ptype parser.
>>>        [1] rte_eth_dev_set_supported_ptypes(*port_id*, RTE_PTYPE_UNKNOWN,
>>> 					  NULL, 0);
>> And for the 7/7 patch, why we are updating all examples, is the API something do
>> we really need to call for any DPDK application? I am for leaving the default
>> behavior unless there is a very specific case for set or disable packet typing.
>> Instead implement a command in testpmd to test this feature.
> 
> If an application does not use packet types provided in mbuf, it is
> better to inform PMD that the information is not required to allow PMD
> to do optimizations. 
> 

I can see disabling packet type detection may increase the performance but
sample applications are to demonstrate a specific feature, adding these kind of
APIs will pollute them.
'skeleton' app that shows the most basic code for forwarding sample, why it is
now having "experimental" 'set_supported_ptypes()' API? Same for other. As said
before I think a testpmd command suits better here.


> Yes, may be it would be better to have it as the
> default behaviour, but it would be behaviour change in comparison
> to previous DPDK releases and it is better to avoid it.

Sorry I missed why not calling this function cause behavior change? I think it
is other way around, no?


> Thanks,
> Andrew.
> 
>>> v12 Changes:
>>> -----------
>>> - Rebase onto next-net.
>>>
>>> v11 Changes:
>>> -----------
>>> - Use RTE_DIM to get array size.
>>> - Since we are using a list of MASKs to validate ptype_mask return -EINVAL
>>>    if any unknown mask is set.
>>> - Rebase to TOT.
>>>
>>> v10 Changes:
>>> -----------
>>> - Modify ptype_mask validation in set_supported_ptypes.(Andrew)
>>>
>>> v9 Changes:
>>> ----------
>>> - Add ptype_mask validation in set_supported_ptypes.(Andrew)
>>> - Make description more verbose.
>>>
>>> v8 Changes:
>>> ----------
>>> - Make description more verbose.
>>> - Set RTE_PTYPE_UNKNOWN in set_ptypes array when either get ot set ptypes
>>>    is not supported by ethernet device.
>>>
>>> v7 Changes:
>>> ----------
>>> - Fix unused variable in net/octeontx2
>>>
>>> v6 Changes:
>>> ----------
>>> - Add additional checks for set supported ptypes.(Andrew)
>>> - Clarify `rte_eth_dev_set_supported_ptypes` documentation.
>>> - Remove DEV_RX_OFFLOAD_FLOW_MARK emulation from net/octeontx2.
>>>
>>> v5 Changes:
>>> ----------
>>> - Fix typos.
>>>
>>> v4 Changes:
>>> ----------
>>> - Set the last element in set_ptype array as RTE_PTYPE_UNKNOWN to mark the end
>>>    of array.
>>> - Fix invalid set ptype function call in examples.
>>> - Remove setting rte_eth_dev_set_supported_ptypes to UNKNOWN in l3fwd-power.
>>>
>>> v3 Changes:
>>> ----------
>>> - Add missing release notes. (Andrew)
>>> - Re-word various descriptions.
>>> - Fix ptype set logic.
>>>
>>> v2 Changes:
>>> ----------
>>> - Update release notes. (Andrew)
>>> - Redo commit logs. (Andrew)
>>> - Disable ptype parsing for unsupported examples. (Jerin)
>>> - Disable RSS write only in generic mode eventdev_pipeline. (Jerin)
>>> - Modify set_supported_ptypes function to return successfuly set mask
>>>    instead of failure.
>>> - Dropped set_supported_ptypes to drivers by handling in library
>>>    layer, interested PMD can add it in.
>>>
>>> Pavan Nikhilesh (7):
>>>    ethdev: add set ptype function
>>>    ethdev: add mbuf RSS update as an offload
>>>    ethdev: add flow action type update as an offload
>>>    drivers/net: update Rx RSS hash offload capabilities
>>>    drivers/net: update Rx flow flag and mark capabilities
>>>    examples/eventdev_pipeline: add new Rx RSS hash offload
>>>    examples: disable Rx packet type parsing
>> <...>
>