netdev - Re: [PATCH net-next] ip_gre: remove CRC flag from dev features in gre_gso

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Message-ID: <20201117162952.29c1a699@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net>
Date:   Tue, 17 Nov 2020 16:29:52 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        davem@...emloft.net, gnault@...hat.com, pabeni@...hat.com,
        lorenzo@...nel.org
Subject: Re: [PATCH net-next] ip_gre: remove CRC flag from dev features in
 gre_gso_segment

On Mon, 16 Nov 2020 17:15:47 +0800 Xin Long wrote:
> This patch is to let it always do CRC checksum in sctp_gso_segment()
> by removing CRC flag from the dev features in gre_gso_segment() for
> SCTP over GRE, just as it does in Commit 527beb8ef9c0 ("udp: support
> sctp over udp in skb_udp_tunnel_segment") for SCTP over UDP.
> 
> It could set csum/csum_start in GSO CB properly in sctp_gso_segment()
> after that commit, so it would do checksum with gso_make_checksum()
> in gre_gso_segment(), and Commit 622e32b7d4a6 ("net: gre: recompute
> gre csum for sctp over gre tunnels") can be reverted now.
> 
> Signed-off-by: Xin Long <lucien.xin@...il.com>

Makes sense, but does GRE tunnels don't always have a csum.

Is the current hardware not capable of generating CRC csums over
encapsulated patches at all?

I guess UDP tunnels can be configured without the csums as well 
so the situation isn't much different.

> diff --git a/net/ipv4/gre_offload.c b/net/ipv4/gre_offload.c
> index e0a2465..a5935d4 100644
> --- a/net/ipv4/gre_offload.c
> +++ b/net/ipv4/gre_offload.c
> @@ -15,12 +15,12 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
>  				       netdev_features_t features)
>  {
>  	int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
> -	bool need_csum, need_recompute_csum, gso_partial;
>  	struct sk_buff *segs = ERR_PTR(-EINVAL);
>  	u16 mac_offset = skb->mac_header;
>  	__be16 protocol = skb->protocol;
>  	u16 mac_len = skb->mac_len;
>  	int gre_offset, outer_hlen;
> +	bool need_csum, gso_partial;

Nit, rev xmas tree looks broken now.

>  	if (!skb->encapsulation)
>  		goto out;
> @@ -41,10 +41,10 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
>  	skb->protocol = skb->inner_protocol;
>  
>  	need_csum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_GRE_CSUM);
> -	need_recompute_csum = skb->csum_not_inet;
>  	skb->encap_hdr_csum = need_csum;
>  
>  	features &= skb->dev->hw_enc_features;
> +	features &= ~NETIF_F_SCTP_CRC;
>  
>  	/* segment inner packet. */
>  	segs = skb_mac_gso_segment(skb, features);
> @@ -99,15 +99,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
>  		}
>  
>  		*(pcsum + 1) = 0;
> -		if (need_recompute_csum && !skb_is_gso(skb)) {
> -			__wsum csum;
> -
> -			csum = skb_checksum(skb, gre_offset,
> -					    skb->len - gre_offset, 0);
> -			*pcsum = csum_fold(csum);
> -		} else {
> -			*pcsum = gso_make_checksum(skb, 0);
> -		}
> +		*pcsum = gso_make_checksum(skb, 0);
>  	} while ((skb = skb->next));
>  out:
>  	return segs;