Re: [PATCHv3 net-next 1/2] net: support ip generic csum processing in skb_csum_hwoffload_help

NETIF_F_IP|IPV6_CSUM feature flag indicates UDP and TCP csum offload
while NETIF_F_HW_CSUM feature flag indicates ip generic csum offload
for HW, which includes not only for TCP/UDP csum, but also for other
protocols' csum like GRE's.

However, in skb_csum_hwoffload_help() it only checks features against
NETIF_F_CSUM_MASK(NETIF_F_HW|IP|IPV6_CSUM). So if it's a non TCP/UDP
packet and the features doesn't support NETIF_F_HW_CSUM, but supports
NETIF_F_IP|IPV6_CSUM only, it would still return 0 and leave the HW
to do csum.

This patch is to support ip generic csum processing by checking
NETIF_F_HW_CSUM for all protocols, and check (NETIF_F_IP_CSUM |
NETIF_F_IPV6_CSUM) only for TCP and UDP.

Note that we're using skb->csum_offset to check if it's a TCP/UDP
proctol, this might be fragile. However, as Alex said, for now we
only have a few L4 protocols that are requesting Tx csum offload,
we'd better fix this until a new protocol comes with a same csum
offset.

v1->v2:
  - not extend skb->csum_not_inet, but use skb->csum_offset to tell
    if it's an UDP/TCP csum packet.
v2->v3:
  - add a note in the changelog, as Willem suggested.

Suggested-by: Alexander Duyck <redacted>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/core/dev.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 6df3f1b..aae116d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -3621,7 +3621,18 @@ int skb_csum_hwoffload_help(struct sk_buff *skb,
                return !!(features & NETIF_F_SCTP_CRC) ? 0 :
                        skb_crc32c_csum_help(skb);

-       return !!(features & NETIF_F_CSUM_MASK) ? 0 : skb_checksum_help(skb);
+       if (features & NETIF_F_HW_CSUM)
+               return 0;
+
+       if (features & (NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM)) {