netdev - Re: [PATCH net-next 1/8] net: local checksum offload for encapsulation

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Message-ID: <56A9BD8E.4030209@gmail.com>
Date:	Thu, 28 Jan 2016 15:04:46 +0800
From:	Zang MingJie <zealot0630@...il.com>
To:	netdev@...r.kernel.org
Cc:	netdev@...r.kernel.org, linux-net-drivers@...arflare.com,
	tom@...bertland.com, alexander.duyck@...il.com
Subject: Re: [PATCH net-next 1/8] net: local checksum offload for
 encapsulation

I have also noticed that for gso, all gso segs will have exactly same 
outer udp checksum, this is also because inner checksum cancellation.

Can we also optimize that outer udp checksum should be only calculated 
once for all gso segs ?

On 01/09/2016 03:45 AM, Edward Cree wrote:
> The arithmetic properties of the ones-complement checksum mean that a
>   correctly checksummed inner packet, including its checksum, has a ones
>   complement sum depending only on whatever value was used to initialise
>   the checksum field before checksumming (in the case of TCP and UDP,
>   this is the ones complement sum of the pseudo header, complemented).
> Consequently, if we are going to offload the inner checksum with
>   CHECKSUM_PARTIAL, we can compute the outer checksum based only on the
>   packed data not covered by the inner checksum, and the initial value of
>   the inner checksum field.
>
> Signed-off-by: Edward Cree <ecree@...arflare.com>
> ---
>   include/linux/skbuff.h  | 24 ++++++++++++++++++++++++
>   net/ipv4/udp.c          | 20 ++++++++++----------
>   net/ipv6/ip6_checksum.c | 14 +++++++-------
>   3 files changed, 41 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 6b6bd42..ee063ff 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3665,5 +3665,29 @@ static inline unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>   	return hdr_len + skb_gso_transport_seglen(skb);
>   }
>
> +/* Local Checksum Offload.
> + * Compute outer checksum based on the assumption that the
> + * inner checksum will be offloaded later.
> + * Fill in outer checksum adjustment (e.g. with sum of outer
> + * pseudo-header) before calling.
> + * Also ensure that inner checksum is in linear data area.
> + */
> +static inline __wsum lco_csum(struct sk_buff *skb)
> +{
> +	char *inner_csum_field;
> +	__wsum csum;
> +
> +	/* Start with complement of inner checksum adjustment */
> +	inner_csum_field = skb->data + skb_checksum_start_offset(skb) +
> +				skb->csum_offset;
> +	csum = ~csum_unfold(*(__force __sum16 *)inner_csum_field);
> +	/* Add in checksum of our headers (incl. outer checksum
> +	 * adjustment filled in by caller)
> +	 */
> +	csum = skb_checksum(skb, 0, skb_checksum_start_offset(skb), csum);
> +	/* The result is the checksum from skb->data to end of packet */
> +	return csum;
> +}
> +
>   #endif	/* __KERNEL__ */
>   #endif	/* _LINUX_SKBUFF_H */
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 8841e98..f9bd5db 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -767,16 +767,18 @@ void udp_set_csum(bool nocheck, struct sk_buff *skb,
>   {
>   	struct udphdr *uh = udp_hdr(skb);
>
> -	if (nocheck)
> +	if (nocheck) {
>   		uh->check = 0;
> -	else if (skb_is_gso(skb))
> +	} else if (skb_is_gso(skb)) {
>   		uh->check = ~udp_v4_check(len, saddr, daddr, 0);
> -	else if (skb_dst(skb) && skb_dst(skb)->dev &&
> -		 (skb_dst(skb)->dev->features &
> -		  (NETIF_F_IP_CSUM | NETIF_F_HW_CSUM))) {
> -
> -		BUG_ON(skb->ip_summed == CHECKSUM_PARTIAL);
> -
> +	} else if (skb->ip_summed == CHECKSUM_PARTIAL) {
> +		uh->check = 0;
> +		uh->check = ~udp_v4_check(len, saddr, daddr, lco_csum(skb));
> +		if (uh->check == 0)
> +			uh->check = CSUM_MANGLED_0;
> +	} else if (skb_dst(skb) && skb_dst(skb)->dev &&
> +		   (skb_dst(skb)->dev->features &
> +		    (NETIF_F_IP_CSUM | NETIF_F_HW_CSUM))) {
>   		skb->ip_summed = CHECKSUM_PARTIAL;
>   		skb->csum_start = skb_transport_header(skb) - skb->head;
>   		skb->csum_offset = offsetof(struct udphdr, check);
> @@ -784,8 +786,6 @@ void udp_set_csum(bool nocheck, struct sk_buff *skb,
>   	} else {
>   		__wsum csum;
>
> -		BUG_ON(skb->ip_summed == CHECKSUM_PARTIAL);
> -
>   		uh->check = 0;
>   		csum = skb_checksum(skb, 0, len, 0);
>   		uh->check = udp_v4_check(len, saddr, daddr, csum);
> diff --git a/net/ipv6/ip6_checksum.c b/net/ipv6/ip6_checksum.c
> index 9a4d732..eba525d 100644
> --- a/net/ipv6/ip6_checksum.c
> +++ b/net/ipv6/ip6_checksum.c
> @@ -98,11 +98,13 @@ void udp6_set_csum(bool nocheck, struct sk_buff *skb,
>   		uh->check = 0;
>   	else if (skb_is_gso(skb))
>   		uh->check = ~udp_v6_check(len, saddr, daddr, 0);
> -	else if (skb_dst(skb) && skb_dst(skb)->dev &&
> -		 (skb_dst(skb)->dev->features & NETIF_F_IPV6_CSUM)) {
> -
> -		BUG_ON(skb->ip_summed == CHECKSUM_PARTIAL);
> -
> +	else if (skb->ip_summed == CHECKSUM_PARTIAL) {
> +		uh->check = 0;
> +		uh->check = ~udp_v6_check(len, saddr, daddr, csum_fold(lco_csum(skb));
> +		if (uh->check == 0)
> +			uh->check = CSUM_MANGLED_0;
> +	} else if (skb_dst(skb) && skb_dst(skb)->dev &&
> +		   (skb_dst(skb)->dev->features & NETIF_F_IPV6_CSUM) {
>   		skb->ip_summed = CHECKSUM_PARTIAL;
>   		skb->csum_start = skb_transport_header(skb) - skb->head;
>   		skb->csum_offset = offsetof(struct udphdr, check);
> @@ -110,8 +112,6 @@ void udp6_set_csum(bool nocheck, struct sk_buff *skb,
>   	} else {
>   		__wsum csum;
>
> -		BUG_ON(skb->ip_summed == CHECKSUM_PARTIAL);
> -
>   		uh->check = 0;
>   		csum = skb_checksum(skb, 0, len, 0);
>   		uh->check = udp_v6_check(len, saddr, daddr, csum);
>
>