netdev - Re: [PATCH v2 net 2/2] net: core: generic XDP support for stacked device

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Date:   Tue, 21 May 2019 13:54:26 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        xdp-newbies@...r.kernel.org, bpf@...r.kernel.org,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH v2 net 2/2] net: core: generic XDP support for stacked
 device


On 2019/5/20 下午11:53, Stephen Hemminger wrote:
> On Mon, 20 May 2019 11:11:05 +0200
> Jiri Pirko <jiri@...nulli.us> wrote:
>
>> Sun, May 19, 2019 at 05:10:46AM CEST, stephen@...workplumber.org wrote:
>>> When a device is stacked like (team, bonding, failsafe or netvsc) the
>>> XDP generic program for the parent device is not called.  In these
>>> cases, the rx handler changes skb->dev to its own in the receive
>>> handler, and returns RX_HANDLER_ANOTHER.  Fix this by calling
>>> do_xdp_generic if necessary before starting another round.
>>>
>>> Review of all the places RX_HANDLER_ANOTHER is returned
>>> show that the current devices do correctly change skb->dev.
>>>
>>> There was an older patch that got abandoned that did the
>>> same thing, this is just a rewrite.
>>>
>>> Suggested-by: Jason Wang <jasowang@...hat.com>
>>> Fixes: d445516966dc ("net: xdp: support xdp generic on virtual devices")
>>> Signed-off-by: Stephen Hemminger <sthemmin@...rosoft.com>
>>> Acked-by: Jason Wang <jasowang@...hat.com>
>>> ---
>> I'm always scarred of changes like this. The history tells us that this
>> codepaths are very fragile. It took us non-trivial efford to fix bonding
>> here, not to mention vlans (that was pain).
>>
>> The reason for troubles was often fact that different flows were treated
>> differently (vlan accel/non-accel).
> Yes, this is a sensitive path. Another alternative is to fix it
> inside each device (netvsc). That is what my earlier patch did and that
> is what is being done now (probably will make it into the RHEL on Azure
> drivers).
>   
>> This patch calls do_xdp_generic for master device in different point in
>> the receive patch comparing to lower device. Would it be possible to
>> unify this? E.g. by moving do_xdp_generice() call from
>> netif_rx_internal()/netif_receive_skb_internal() here,
>> to the beginning of __netif_receive_skb_core()?
>>
> That could work, but has the question about doing XDP farther down
> call stack (lower performance).
>
> There is also the case what if both paths support XDP in driver.
> This would be the ideal case, how would this work?
>
>

I think we have a clear request of allowing native XDP to work on 
stacked device. We're missing some generic building blocks (like XDP rx 
handler) here.

Thanks