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

On Thu, Jan 28, 2021 at 6:07 AM Willem de Bruijn
[off-list ref] wrote:

On Thu, Jan 28, 2021 at 4:29 AM Xin Long [off-list ref] wrote:

quoted

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)) {

Should this check the specific feature flag against skb->protocol? I
don't know if there are actually instances that only support one of
the two flags.

The issue is at a certain point we start excluding devices that were
previously working.

All this patch is really doing is using the checksum offset to
identify the cases that were previously UDP or TCP offloads and
letting those through with the legacy path, while any offsets that are
not those two, such as the GRE checksum will now have to be explicitly
caught by the NETIF_F_HW_CSUM case and not accepted by the other
cases.