Re: [dpdk-dev] [PATCH v15 3/7] ethdev: add validation to offloads set by PMD

[Matan Azrad <matan@mellanox.com>]

From: Andrew Rybchenko
> On 11/3/19 6:16 PM, Matan Azrad wrote
> > From: Andrew Rybchenko
> >> On 11/3/19 9:57 AM, Matan Azrad wrote:
> >>> Hi
> >>>
> >>> From: Andrew Rybchenko
> >>>> On 10/31/19 7:33 PM, Pavan Nikhilesh Bhagavatula wrote:
> >>>>>> From: Pavan Nikhilesh Bhagavatula
> >>>>>>> Hi Matan,
> >>>>>>>
> >>>>>>>> Hi Pavan
> >>>>>>>>
> >>>>>>>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
> >>>>>>>>> Some PMDs cannot work when certain offloads are
> >> enable/disabled,
> >>>>>>>>> as a workaround PMDs auto enable/disable offloads internally
> >>>>>>>>> and expose it through dev->data->dev_conf.rxmode.offloads.
> >>>>>>>>>
> >>>>>>>>> After device specific dev_configure is called compare the
> >>>>>>>>> requested offloads to the offloads exposed by the PMD and, if
> >>>>>>>>> the PMD failed to enable a given offload then log it and
> >>>>>>>>> return -EINVAL from rte_eth_dev_configure, else if the PMD
> >>>>>>>>> failed to disable a given offload log and continue with
> >> rte_eth_dev_configure.
> >>>>>>>>>
> >>>>>>>> rte_eth_dev_configure can be called more than 1 time in the
> >>>>>>>> device life time, How can you know what is the minimum offload
> >>>>>>>> configurations required by the port after the first call?
> >>>>>>>> Maybe putting it in dev info is better, what do you think?
> >>>>>>>>
> >>>>>>> We only return -EINVAL in the case where we enable an offload
> >>>>>>> advertised by dev_info and the port still fails to enable it.
> >>>>>> Are you sure it is ok that devices may disable\enable offloads
> >>>>>> under the hood without user notification?
> >>>>> Some devices already do it. The above check adds validation for the
> same.
> >>>> The problem is that some offloads cannot be disabled.
> >>> Yes, I understand it.
> >>>
> >>>> If application does not request Rx checksum offload since it does
> >>>> use it, it is not a problem to report it.
> >>> Yes, for RX checksum I tend to agree that application doesn't care
> >>> if the
> >> PMD will calculate the checksum in spite of the offload is disabled.
> >>>
> >>> But what's about other offloads:
> >>> For example in RX: LRO, CRC_KEEP, VLAN_STRIP, JUMBO If the PMD will
> >>> stay them on while the app is disabling it, It can cause a problems
> >>> to the
> >> application (affects the packet length).
> >>
> >> Yes, I agree that some offloads are critical to be disabled, but
> >> RSS_HASH discussed in the changeset is not critical.
> >
> > So, are you agree It should not be checked globally for all the offloads in
> ethdev layer?
> 
> If offload is not requested, but enabled (since PMD cannot disable it), right
> not it will not fail configure, but warn about it in logs.
> 

In this case warning print is not enough since it can be critical for the application for some offloads.
It can be very weird for the application to see that some offload are on while the application doesn't expect them to be on.
it even can cause app crash(at least for the RX offload I wrote above).


> > It even be more problematic if the dynamic offload field in mbuf is not exist
> at all.

Any answer here?

> >>
> >>> For example in TX: TSO, VLAN, MULTI_SEG.....
> >>
> >> Tx is not that critical since application should not request these
> >> offloads per- packet. Tx offloads are mainly required to ensure that
> >> application may request the offload per packet and it will be done.
> >
> > yes, you right, In TX it looks less critical (for now).
> >
> >>
> >>>> Of course, it could be a problem if the offload is used, but
> >>>> application wants to disable it, for example, for debugging purposes.
> >>>> In this case, the solution is to mask offloads on application
> >>>> level, which is not ideal as well.
> >>> Why not ideal?
> >>
> >> It eats CPU cycles.
> >
> > Sorry, I don't understand your use case here.
> 
> If application wants to try code path without, for example, Rx checksum
> offload, it could be insufficient to disable the offload right now, but also
> required to cleanup offload results flags in each mbuf (if PMD does not
> support the offload disabling).

What is "right now"? Configuration time?

If application will know that PMD cannot disable the rx-checksum in configuration time,
It can plan to not clean this flag in mbuf for each rx mbuf.


It looks me like PMD limitation which can be solved by 2 options:
1. Capability information which say to the app what offload may not be disabled.
2. Add limitation in the PMD documentation and print warning\error massage from the PMD.
 
> >>> If application can know the limitation of offloads disabling (for
> >>> example to
> >> read capability on it)
> >>> The application has all information to take decisions.
> >>>
> >>>> Anyway, the patch just tries to highlight difference of applied
> >>>> from requested. So, it is a step forward.
> >>>> Also, the patch will fail configure if an offload is requested, but
> >>>> not
> >> enabled.
> >>>>
> >>>>>> Can't it break applications?
> >>>>>> Why does the device expose unsupported offloads in dev info?
> >>>>>> Does it update the running offload usynchronically? Race?
> >>>>>> Can you explain also your specific use case?
> >>>>>>
> >>>>>>
> >>>>>>>> Matan
> >