netdev - Re: [PATCH net-next 2/3] net: add CSUM_T_IP_GENERIC csum

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Date:   Fri, 22 Jan 2021 11:18:26 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Davide Caratti <dcaratti@...hat.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next 2/3] net: add CSUM_T_IP_GENERIC csum_type

On Fri, Jan 22, 2021 at 2:13 AM Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> On Thu, Jan 21, 2021 at 12:46 AM Xin Long <lucien.xin@...il.com> wrote:
> >
> > This patch is to extend csum_type field to 2 bits, and introduce
> > CSUM_T_IP_GENERIC csum type, and add the support for this in
> > skb_csum_hwoffload_help(), just like CSUM_T_SCTP_CRC.
> >
> > Note here it moves dst_pending_confirm field below ndisc_nodetype
> > to avoid a memory hole.
> >
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  include/linux/skbuff.h |  5 +++--
> >  net/core/dev.c         | 17 +++++++++++++----
> >  2 files changed, 16 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 67b0a01..d5011fb 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -224,6 +224,7 @@
> >
> >  #define CSUM_T_INET            0
> >  #define CSUM_T_SCTP_CRC                1
> > +#define CSUM_T_IP_GENERIC      2
> >
> >  /* Maximum value in skb->csum_level */
> >  #define SKB_MAX_CSUM_LEVEL     3
> > @@ -839,11 +840,11 @@ struct sk_buff {
> >         __u8                    vlan_present:1;
> >         __u8                    csum_complete_sw:1;
> >         __u8                    csum_level:2;
> > -       __u8                    csum_type:1;
> > -       __u8                    dst_pending_confirm:1;
> > +       __u8                    csum_type:2;
> >  #ifdef CONFIG_IPV6_NDISC_NODETYPE
> >         __u8                    ndisc_nodetype:2;
> >  #endif
> > +       __u8                    dst_pending_confirm:1;
> >
> >         __u8                    ipvs_property:1;
> >         __u8                    inner_protocol_type:1;
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 3241de2..6d48af2 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -3617,11 +3617,20 @@ static struct sk_buff *validate_xmit_vlan(struct sk_buff *skb,
> >  int skb_csum_hwoffload_help(struct sk_buff *skb,
> >                             const netdev_features_t features)
> >  {
> > -       if (unlikely(skb_csum_is_sctp(skb)))
> > -               return !!(features & NETIF_F_SCTP_CRC) ? 0 :
> > -                       skb_crc32c_csum_help(skb);
> > +       if (likely(!skb->csum_type))
> > +               return !!(features & NETIF_F_CSUM_MASK) ? 0 :
> > +                      skb_checksum_help(skb);
> >
> > -       return !!(features & NETIF_F_CSUM_MASK) ? 0 : skb_checksum_help(skb);
> > +       if (skb_csum_is_sctp(skb)) {
> > +               return !!(features & NETIF_F_SCTP_CRC) ? 0 :
> > +                      skb_crc32c_csum_help(skb);
> > +       } else if (skb->csum_type == CSUM_T_IP_GENERIC) {
> > +               return !!(features & NETIF_F_HW_CSUM) ? 0 :
> > +                      skb_checksum_help(skb);
> > +       } else {
> > +               pr_warn("Wrong csum type: %d\n", skb->csum_type);
> > +               return 1;
> > +       }
>
> Is the only difference between CSUM_T_IP_GENERIC the fact that we
> check for NETIF_F_HW_CSUM versus using NETIF_F_CSUM_MASK? If so I
> don't think adding the new bit is adding all that much value. Instead
> you could probably just catch this in the testing logic here.
>
> You could very easily just fold CSUM_T_IP_GENERIC into CSUM_T_INET,
> and then in the checks here you split up the checks for
> NETIF_F_HW_CSUM as follows:
If so, better not to touch csum_not_inet now. I will drop the patch 1/3.

>
>  if (skb_csum_is_sctp(skb))
>     return !!(features & NETIF_F_SCTP_CRC) ? 0 : skb_crc32c_csum_help(skb);
>
> if (skb->csum_type) {
>     pr_warn("Wrong csum type: %d\n", skb->csum_type);
>     return 1;
> }
>
> if (features & NETIF_F_HW_CSUM)
>     return 0;
>
> if (features & NETIF_F_CSUM_MASK) {
>     switch (skb->csum_offset) {
>     case offsetof(struct tcphdr, check):
>     case offsetof(struct udphdr, check):
>             return 0;
>     }
Question is: is it reliable to check the type by skb->csum_offset?
What if one day there's another protocol, whose the checksum field
is on the same offset, which is also using the CSUM_T_IP_GENERIC?

> }
>
> return skb_checksum_help(skb);