Re: [dpdk-dev] [PATCH] ethdev: add flow tag

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

On Thu, Jul 04, 2019 at 04:23:02PM -0700, Yongseok Koh wrote:
> A tag is a transient data which can be used during flow match. This can be
> used to store match result from a previous table so that the same pattern
> need not be matched again on the next table. Even if outer header is
> decapsulated on the previous match, the match result can be kept.
> 
> Some device expose internal registers of its flow processing pipeline and
> those registers are quite useful for stateful connection tracking as it
> keeps status of flow matching. Multiple tags are supported by specifying
> index.
> 
> Example testpmd commands are:
> 
>   flow create 0 ingress pattern ... / end
>     actions set_tag index 2 value 0xaa00bb mask 0xffff00ff /
>             set_tag index 3 value 0x123456 mask 0xffffff /
>             vxlan_decap / jump group 1 / end
> 
>   flow create 0 ingress pattern ... / end
>     actions set_tag index 2 value 0xcc00 mask 0xff00 /
>             set_tag index 3 value 0x123456 mask 0xffffff /
>             vxlan_decap / jump group 1 / end
> 
>   flow create 0 ingress group 1
>     pattern tag index is 2 value spec 0xaa00bb value mask 0xffff00ff /
>             eth ... / end
>     actions ... jump group 2 / end
> 
>   flow create 0 ingress group 1
>     pattern tag index is 2 value spec 0xcc00 value mask 0xff00 /
>             tag index is 3 value spec 0x123456 value mask 0xffffff /
>             eth ... / end
>     actions ... / end
> 
>   flow create 0 ingress group 2
>     pattern tag index is 3 value spec 0x123456 value mask 0xffffff /
>             eth ... / end
>     actions ... / end
> 
> Signed-off-by: Yongseok Koh <yskoh@mellanox.com>

Hi Yongseok,

Only high level questions for now, while it unquestionably looks useful,
from a user standpoint exposing the separate index seems redundant and not
necessarily convenient. Using the following example to illustrate:

 actions set_tag index 3 value 0x123456 mask 0xfffff

 pattern tag index is 3 value spec 0x123456 value mask 0xffffff

I might be missing something, but why isn't this enough:

 pattern tag index is 3 # match whatever is stored at index 3

Assuming it can work, then why bother with providing value spec/mask on
set_tag? A flow rule pattern matches something, sets some arbitrary tag to
be matched by a subsequent flow rule and that's it. It even seems like
relying on the index only on both occasions is enough for identification.

Same question for the opposite approach; relying on the value, never
mentioning the index.

I'm under the impression that the index is a hardware-specific constraint
that shouldn't be exposed (especially since it's an 8-bit field). If so, a
PMD could keep track of used indices without having them exposed through the
public API.

-- 
Adrien Mazarguil
6WIND