netdev - Re: [RFC] net: store port/representative id in metadata

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Date:   Fri, 23 Sep 2016 21:45:44 +0100
From:   Jakub Kicinski <kubakici@...pl>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Jiri Benc <jbenc@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        netdev@...r.kernel.org, Thomas Graf <tgraf@...g.ch>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        ogerlitz@...lanox.com, ast@...nel.org, daniel@...earbox.net,
        simon.horman@...ronome.com, Paolo Abeni <pabeni@...hat.com>,
        Pravin B Shelar <pshelar@...ira.com>,
        hannes@...essinduktion.org
Subject: Re: [RFC] net: store port/representative id in metadata_dst

On Fri, 23 Sep 2016 13:25:10 -0700, John Fastabend wrote:
> On 16-09-23 01:17 PM, Jakub Kicinski wrote:
> > On Fri, 23 Sep 2016 10:22:59 -0700, Samudrala, Sridhar wrote:  
> >> On 9/23/2016 8:29 AM, Jakub Kicinski wrote:  
>  [...]  
>  [...]  
> >>
> >> The 'accel' parameter in dev_queue_xmit_accel() is currently only passed
> >> to ndo_select_queue() via netdev_pick_tx() and is used to select the tx 
> >> queue.
> >> Also, it is not passed all the way to the driver specific xmit routine.  
> >> Doesn't it require
> >> changing all the driver xmit routines if we want to pass this parameter?
> >>  
>  [...]  
> >>
> >> Yes.  The VFPR netdevs don't have any HW queues associated with them and 
> >> we would like
> >> to use the PF queues for the xmit.
> >> I was also looking into some way of passing the port id via skb 
> >> parameter to the
> >> dev_queue_xmit() call so that the PF xmit routine can do a directed 
> >> transmit to a specifc VF.
> >> Is skb->cb an option to pass this info?
> >> dst_metadata approach would work  too if it is acceptable.  
> > 
> > I don't think we can trust skb->cb to be set to anything meaningful
> > when the skb is received by the lower device. 
> 
> Agreed. I wouldn't recommend using skb->cb. How about passing it through
> dev_queue_xmit_accel() through to the driver?
> 
> If you pass the metadata through the dev_queue_xmit_accel() handle tx
> queue  selection would work using normal mechanisms (xps, select_queue,
> cls  hook, etc.). If you wanted to pick some specific queue based on
> policy the policy could be loaded into one of those hooks.

Do you mean without extending how accel is handled by
dev_queue_xmit_accel() today?  If my goal is to not have extra HW
queues then I don't see how I could mux in the lower dev without extra
locking (as I tried to explain two emails ago).  Sorry for being slow
here :(