Date:   Tue, 27 Jul 2021 12:13:37 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Vlad Buslov <vladbu@...dia.com>,
        Simon Horman <simon.horman@...igine.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...lanox.com>, netdev@...r.kernel.org,
        oss-drivers@...igine.com, Baowen Zheng <baowen.zheng@...igine.com>,
        Louis Peens <louis.peens@...igine.com>
Subject: Re: [PATCH net-next 1/3] flow_offload: allow user to offload tc
 action to net device

On 2021-07-27 10:38 a.m., Vlad Buslov wrote:
> 
> On Tue 27 Jul 2021 at 16:04, Simon Horman <simon.horman@...igine.com> wrote:

>>>
>>> Also showing a tc command line in the cover letter on how one would
>>> ask for a specific action to be offloaded.
>>
>> In practice actions are offloaded when a flow using them is offloaded.
>> So I think we need to consider what the meaning of IN_HW is.
>>
>> Is it that:
>>
>> * The driver (and potentially hardware, though not in our current
>>    implementation) has accepted the action for offload;
>> * That a classifier that uses the action has bee offloaded;
>> * Or something else?
> 
> I think we have the same issue with filters - they might not be in
> hardware after driver callback returned "success" (due to neigh state
> being invalid for tunnel_key encap, for example).
> 

Sounds like we need another state for this. Otherwise, how do you debug
that something is sitting in the driver and not in hardware after you
issued a command to offload it? How do i tell today?
Also knowing reason why something is sitting in the driver would be
helpful.

>> With regards to a counter, I'm not quite sure what this would be:
>>
>> * The number of devices where the action has been offloaded (which ties
>>    into the question of what we mean by IN_HW)
>> * The number of offloaded classifier instances using the action
>> * Something else
> 
> I would prefer to have semantics similar to filters:
> 
> 1. Count number of driver callbacks that returned "success".
> 
> 2. If count > 0, then set in_hw flag.
> 
> 3. Set in_hw_count to success count.
> 
> This would allow user to immediately determine whether action passed
> driver validation.
>

I didnt follow this:
Are we refering to the the "block" semantics (where a filter for
example applies to multiple devices)?

>>
>> Regarding a flag to control offload:
>>
>> * For classifiers (at least the flower classifier) there is the skip_sw and
>>    skip_hw flags, which allow control of placement of a classifier in SW and
>>    HW.
>> * We could add similar flags for actions, which at least in my
>>    world view would have the net-effect of controlling which classifiers can
>>    be added to sw and hw - f.e. a classifier that uses an action marked
>>    skip_hw could not be added to HW.

I guess it depends on the hardware implementation.
In S/W we have two modes:
Approach A: create an action and then 2) bind it to a filter.
Approach B: Create a filter and then bind it to an action.

And #2A can be repeated multiple times for the same action
(would require some index as a reference for the action)
To Simon's comment above that would mean allowing
"a classifier that uses an action marked skip_hw to be added to HW"
i.e
Some hardware is capable of doing both option #A and #B.

Todays offload assumes #B - in which both filter and action are assumed
offloaded.

I am hoping whatever approach we end up agreeing on doesnt limit
either mode.

>> * Doing so would add some extra complexity and its not immediately apparent
>>    to me what the use-case would be given that there are already flags for
>>    classifiers.
> Yeah, adding such flag for action offload seems to complicate things.
> Also, "skip_sw" flag doesn't even make much sense for actions. I thought
> that "skip_hw" flag would be nice to have for users that would like to
> avoid "spamming" their NIC drivers (potentially causing higher latency
> and resource consumption) for filters/actions they have no intention to
> offload to hardware, but I'm not sure how useful is that option really
> is.

Hold on Vlad.
So you are looking at this mostly as an optimization to speed up h/w
control updates? ;->

I was looking at it more as a (currently missing) feature improvement.
We already have a use case that is implemented by s/w today. The feature
mimics it in h/w.

At minimal all existing NICs should be able to support the counters
as mapped to simple actions like drop. I understand for example if some
cant support adding separately offloading of tunnels for example.
So the syntax is something along the lines of:

tc actions add action drop index 15 skip_sw
tc filter add dev ...parent ... protocol ip prio X ..\
u32/flower skip_sw match ... flowid 1:10 action gact index 15

You get an error if counter index 15 is not offloaded or
if skip_sw was left out..

And then later on, if you support sharing of actions:
tc filter add dev ...parent ... protocol ip prio X2 ..\
u32/flower skip_sw match ... flowid 1:10 action gact index 15

cheers,
jamal

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