netdev - Re: [PATCH net-next 1/2] net: support ip generic csum processing in skb_csum_hwoffload

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Date:   Thu, 28 Jan 2021 17:13:59 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Davide Caratti <dcaratti@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net-next 1/2] net: support ip generic csum processing in skb_csum_hwoffload_help

On Tue, Jan 26, 2021 at 9:59 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Sun, Jan 24, 2021 at 3:47 AM Xin Long <lucien.xin@...il.com> wrote:
> >
> > 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.
> >
> > v1->v2:
> >   - not extend skb->csum_not_inet, but use skb->csum_offset to tell
> >     if it's an UDP/TCP csum packet.
> >
> > Suggested-by: Alexander Duyck <alexander.duyck@...il.com>
> > Signed-off-by: Xin Long <lucien.xin@...il.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)) {
> > +               switch (skb->csum_offset) {
> > +               case offsetof(struct tcphdr, check):
> > +               case offsetof(struct udphdr, check):
>
> This relies on no other protocols requesting CHECKSUM_PARTIAL
> with these csum_offset values.
>
> That is a fragile assumption. It may well be correct, and Alex argues
> that point in v1 of the patch. I think that argumentation at the least
> should be captured as a comment or in the commit message.
will add a note in changelog and repost, thanks!

>
> Or perhaps limit this optimization over s/w checksumming to
>
>   skb->sk &&
>   (skb->sk->sk_family == AF_INET  || .. ) &&
>   (skb->sk->sk_type == SOCK_STREAM || ..)
>
> ?