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

From: Jiri Pirko <[email-masked]>
Date: Mon, 20 May 2019 11:11:05 +0200

Sun, May 19, 2019 at 05:10:46AM CEST, [email-masked] 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 <[email-masked]>
>Fixes: d445516966dc ("net: xdp: support xdp generic on virtual devices")
>Signed-off-by: Stephen Hemminger <[email-masked]>
>Acked-by: Jason Wang <[email-masked]>
>---
> net/core/dev.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
>diff --git a/net/core/dev.c b/net/core/dev.c
>index b6b8505cfb3e..240d0b2de1a8 100644
>--- a/net/core/dev.c
>+++ b/net/core/dev.c
>@@ -4921,6 +4921,16 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
> 			ret = NET_RX_SUCCESS;
> 			goto out;
> 		case RX_HANDLER_ANOTHER:
>+			if (static_branch_unlikely(&generic_xdp_needed_key)) {
>+				struct bpf_prog *xdp_prog;
>+
>+				xdp_prog = rcu_dereference(skb->dev->xdp_prog);
>+				ret = do_xdp_generic(xdp_prog, skb);
>+				if (ret != XDP_PASS) {
>+					ret = NET_RX_SUCCESS;
>+					goto out;
>+				}
>+			}

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).

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()?

> 			goto another_round;
> 		case RX_HANDLER_EXACT:
> 			deliver_exact = true;
>-- 
>2.20.1