Subject: Re: [RFC net-next v1 4/5] net: macsec: introduce mdo_insert_tx_tag

From: Sabrina Dubroca <[email-masked]>
Date: Wed, 16 Aug 2023 22:40:20 +0200

2023-08-11, 18:32:48 +0300, Radu Pirea (NXP OSS) wrote:
> Offloading MACsec in PHYs requires inserting the SecTAG and the ICV in
> the ethernet frame. This operation will increase the frame size with 32
> bytes.

"up to 32 bytes"?

The SecTAG and ICV can both be shorter, at least with the software
implementation.

[...]
> +static struct sk_buff *insert_tx_tag(struct sk_buff *skb,
> +				     struct net_device *dev)
> +{
[...]
> +
> +	ctx.secy = &macsec->secy;
> +	ctx.skb = skb;

I think it would be a bit more readable to just pass the skb to
 ->mdo_insert_tx_tag instead of adding it to the context.

> +
> +	err = ops->mdo_insert_tx_tag(&ctx);
> +	if (err)
> +		goto cleanup;

[...]
> @@ -3403,6 +3470,13 @@ static netdev_tx_t macsec_start_xmit(struct sk_buff *skb,
>  		skb_dst_drop(skb);
>  		dst_hold(&md_dst->dst);
>  		skb_dst_set(skb, &md_dst->dst);
> +
> +		skb = insert_tx_tag(skb, dev);
> +		if (IS_ERR(skb)) {
> +			dev->stats.tx_dropped++;

That should probably use DEV_STATS_INC (see commit
32d0a49d36a2 ("macsec: use DEV_STATS_INC()")).

> +			return NETDEV_TX_OK;
> +		}
> +
>  		skb->dev = macsec->real_dev;
>  		return dev_queue_xmit(skb);
>  	}
> @@ -4137,6 +4211,11 @@ static int macsec_newlink(struct net *net, struct net_device *dev,
>  			if (err)
>  				goto del_dev;
>  		}
> +
> +		dev->needed_headroom -= MACSEC_NEEDED_HEADROOM;
> +		dev->needed_headroom += ops->needed_headroom;
> +		dev->needed_tailroom -= MACSEC_NEEDED_TAILROOM;
> +		dev->needed_tailroom += ops->needed_tailroom;

If the driver doesn't set ops->needed_headroom, we'll subtract
MACSEC_NEEDED_HEADROOM and not add anything back. Is that correct for
all existing drivers? (and same for tailroom)

You set needed_tailroom to 0 in your driver, but the commit message
for this patch says that the HW needs space for the ICV. I'm a bit
puzzled by this, especially since MACSEC_NEEDED_TAILROOM already
reserves space for the ICV.

Also, since this is pattern repeated twice more (with a sign change)
in macsec_update_offload, we could probably stuff this into a helper
(either modifying dev->needed_headroom directly, or returning the
value to add/subtract).

>  	}

[...]
> @@ -302,6 +303,10 @@ struct macsec_ops {
>  	int (*mdo_get_tx_sa_stats)(struct macsec_context *ctx);
>  	int (*mdo_get_rx_sc_stats)(struct macsec_context *ctx);
>  	int (*mdo_get_rx_sa_stats)(struct macsec_context *ctx);
> +	/* Offload tag */
> +	int (*mdo_insert_tx_tag)(struct macsec_context *ctx);
> +	int needed_headroom;
> +	int needed_tailroom;

unsigned?

>  };

-- 
Sabrina