Message-ID: <20210125122724.GA18646@ranger.igk.intel.com>
Date:   Mon, 25 Jan 2021 13:27:24 +0100
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCHv16 bpf-next 3/6] xdp: add a new helper for dev map
 multicast support

On Fri, Jan 22, 2021 at 03:46:49PM +0800, Hangbin Liu wrote:
> This patch is for xdp multicast support. which has been discussed
> before[0], The goal is to be able to implement an OVS-like data plane in
> XDP, i.e., a software switch that can forward XDP frames to multiple ports.
> 
> To achieve this, an application needs to specify a group of interfaces
> to forward a packet to. It is also common to want to exclude one or more
> physical interfaces from the forwarding operation - e.g., to forward a
> packet to all interfaces in the multicast group except the interface it
> arrived on. While this could be done simply by adding more groups, this
> quickly leads to a combinatorial explosion in the number of groups an
> application has to maintain.
> 
> To avoid the combinatorial explosion, we propose to include the ability
> to specify an "exclude group" as part of the forwarding operation. This
> needs to be a group (instead of just a single port index), because a
> physical interface can be part of a logical grouping, such as a bond
> device.
> 
> Thus, the logical forwarding operation becomes a "set difference"
> operation, i.e. "forward to all ports in group A that are not also in
> group B". This series implements such an operation using device maps to
> represent the groups. This means that the XDP program specifies two
> device maps, one containing the list of netdevs to redirect to, and the
> other containing the exclude list.
> 
> To achieve this, I re-implement a new helper bpf_redirect_map_multi()
> to accept two maps, the forwarding map and exclude map. The forwarding
> map could be DEVMAP or DEVMAP_HASH, but the exclude map *must* be
> DEVMAP_HASH to get better performace. If user don't want to use exclude
> map and just want simply stop redirecting back to ingress device, they
> can use flag BPF_F_EXCLUDE_INGRESS.

Hangbin,

before you submit next revision, could you try to apply imperative mood to
your commit messages?

>From Documentation/process/submitting-patches.rst:

<quote>
Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour.
</quote>

That's the thing I'm trying to remind people internally and it feels like
we keep on forgetting about that.

Thanks!

> 
> As both bpf_xdp_redirect_map() and this new helpers are using struct
> bpf_redirect_info, I add a new ex_map and set tgt_value to NULL in the
> new helper to make a difference with bpf_xdp_redirect_map().
> 
> Also I keep the general data path in net/core/filter.c, the native data
> path in kernel/bpf/devmap.c so we can use direct calls to get better
> performace.
> 
> [0] https://xdp-project.net/#Handling-multicast
> 
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
> 

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