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Message-ID: <CALx6S35_-JSix_VjreXbyWCj5jfB-Pu0xaDKzTBNrtkrnijqLA@mail.gmail.com>
Date: Thu, 7 Jan 2016 10:42:26 -0800
From: Tom Herbert <tom@...bertland.com>
To: Edward Cree <ecree@...arflare.com>
Cc: David Miller <davem@...emloft.net>,
Linux Kernel Network Developers <netdev@...r.kernel.org>,
linux-net-drivers@...arflare.com
Subject: Re: [PATCH v2 net-next 1/5] net: local checksum offload for encapsulation
On Thu, Jan 7, 2016 at 9:12 AM, Edward Cree <ecree@...arflare.com> 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 | 26 ++++++++++++++++++++++++++
> net/ipv4/ip_tunnel_core.c | 4 ++++
> net/ipv4/udp.c | 29 ++++++++++-------------------
> net/ipv6/ip6_checksum.c | 24 ++++++++----------------
> 4 files changed, 48 insertions(+), 35 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 6b6bd42..6590d08 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3665,5 +3665,31 @@ 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.
> + * See Documentation/networking/tx-offloads.txt for
> + * explanation of how this works.
> + * 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 outer checksum */
Specifically packet checksum starting from skb->data.
> + return csum;
> +}
> +
> #endif /* __KERNEL__ */
> #endif /* _LINUX_SKBUFF_H */
> diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
> index 1db8418..f39b064 100644
> --- a/net/ipv4/ip_tunnel_core.c
> +++ b/net/ipv4/ip_tunnel_core.c
> @@ -146,6 +146,10 @@ struct metadata_dst *iptunnel_metadata_reply(struct metadata_dst *md,
> }
> EXPORT_SYMBOL_GPL(iptunnel_metadata_reply);
>
> +/* csum_help should only be ever true if the protocol doesn't support LCO.
> + * If the tunnel uses udp_tunnel_xmit_skb(), then it gets LCO for free, and
> + * should always set csum_help to false.
> + */
This comment probably isn't necessary.
> struct sk_buff *iptunnel_handle_offloads(struct sk_buff *skb,
> bool csum_help,
> int gso_type_mask)
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 8841e98..c1c73be 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -767,32 +767,23 @@ 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))) {
> -
I think getting rid of this check is correct, but maybe it should be
in a separate patch?
> - BUG_ON(skb->ip_summed == CHECKSUM_PARTIAL);
> + } else if (skb->ip_summed == CHECKSUM_PARTIAL) {
> + __wsum csum;
>
> + uh->check = ~udp_v4_check(len, saddr, daddr, 0);
> + csum = lco_csum(skb);
> + uh->check = csum_fold(csum);
> + if (uh->check == 0)
> + uh->check = CSUM_MANGLED_0;
> + } else {
> skb->ip_summed = CHECKSUM_PARTIAL;
> skb->csum_start = skb_transport_header(skb) - skb->head;
> skb->csum_offset = offsetof(struct udphdr, check);
> uh->check = ~udp_v4_check(len, saddr, daddr, 0);
> - } 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);
> - if (uh->check == 0)
> - uh->check = CSUM_MANGLED_0;
> -
> - skb->ip_summed = CHECKSUM_UNNECESSARY;
> }
> }
> EXPORT_SYMBOL(udp_set_csum);
> diff --git a/net/ipv6/ip6_checksum.c b/net/ipv6/ip6_checksum.c
> index 9a4d732..19e564a 100644
> --- a/net/ipv6/ip6_checksum.c
> +++ b/net/ipv6/ip6_checksum.c
> @@ -98,27 +98,19 @@ 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) {
> + __wsum csum;
>
> + uh->check = ~udp_v6_check(len, saddr, daddr, 0);
> + csum = lco_csum(skb);
> + uh->check = csum_fold(csum);
Just do uh->check = csum_fold(loc)csum(skb)) to eliminate need for
local csum variable.
> + if (uh->check == 0)
> + uh->check = CSUM_MANGLED_0;
> + } else {
> skb->ip_summed = CHECKSUM_PARTIAL;
> skb->csum_start = skb_transport_header(skb) - skb->head;
> skb->csum_offset = offsetof(struct udphdr, check);
> uh->check = ~udp_v6_check(len, saddr, daddr, 0);
> - } 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);
> - if (uh->check == 0)
> - uh->check = CSUM_MANGLED_0;
> -
> - skb->ip_summed = CHECKSUM_UNNECESSARY;
> }
> }
> EXPORT_SYMBOL(udp6_set_csum);
>
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