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Date:   Tue, 25 Jul 2023 13:47:19 -0700
From:   Joe Damato <jdamato@fastly.com>
To:     Edward Cree <ecree.xilinx@gmail.com>
Cc:     Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org,
        saeedm@nvidia.com, tariqt@nvidia.com, ecree@solarflare.com,
        andrew@lunn.ch, davem@davemloft.net, leon@kernel.org,
        pabeni@redhat.com, bhutchings@solarflare.com, arnd@arndb.de,
        linux-kernel@vger.kernel.org
Subject: Re: [net 0/2] rxfh with custom RSS fixes
Message-ID: <20230725204719.GA1789@fastly.com>
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On Tue, Jul 25, 2023 at 09:40:24AM +0100, Edward Cree wrote:
> On 24/07/2023 23:08, Jakub Kicinski wrote:
> > It'd be great to push the uAPI extensions back and make them
> > netlink-only, but we can't make Joe wait if it takes a long time
> > to finish up the basic conversion :(
> 
> Yeah as I said upthread I don't think we should make Joe wait, if
>  he's got a use case that actually needs it (have you, Joe?  Or
>  is it only GRXFH you need and the investigation just led you to
>  notice SRXFH was broken?)

In short, yes: I'd like to be able to get and set the flow hash keys for
custom RSS contexts on mlx5 which is why I included the patch to mlx5 in
this series... but to be fair I am just one user :) I think it's really
up to you all on the direction you want to go.

Longer story: I am working on building a system which relies on custom RSS
contexts, flow rules to associate flows with RSS contexts (and thus
specific sets of queues), and epoll based busy poll. It's a long story ;)

I had considered changing the flow hash key to see if I could alter the
behavior of the system I am working on, but I ran into both issues
immediately (GRXFH and FLOW_RSS was not supported by mlx5, and SRXFH
was broken) which led me down the path of attempting to fix both so that I
could get and set the flow hash keys. I thought the code might be useful,
so I submit it upstream -- I was not aware of the netlink work (but it
looks really useful!).

It seems that the Mellanox folks are OK with the proposed driver change,
so I am going to send a v2 rebased on net-next with their requested changes.