netdev - Re: CHECKSUM_COMPLETE question

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Date:   Wed, 19 Feb 2020 16:25:46 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Grygorii Strashko <grygorii.strashko@...com>,
        netdev <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Eric Dumazet <edumazet@...gle.com>,
        John Fastabend <john.fastabend@...il.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
Subject: Re: CHECKSUM_COMPLETE question

On Wed, 19 Feb 2020 14:59:16 -0800 Willem de Bruijn wrote:
> On Wed, Feb 19, 2020 at 9:04 AM Grygorii Strashko
> <grygorii.strashko@...com> wrote:
> >
> > Hi All,
> >
> > On 12/02/2020 11:52, Grygorii Strashko wrote:  
> > > Hi All,
> > >
> > > I'd like to ask expert opinion and clarify few points about HW RX checksum offload.
> > >
> > > 1) CHECKSUM_COMPLETE - from description in <linux/skbuff.h>
> > >   * CHECKSUM_COMPLETE:
> > >   *
> > >   *   This is the most generic way. The device supplied checksum of the _whole_
> > >   *   packet as seen by netif_rx() and fills out in skb->csum. Meaning, the
> > >   *   hardware doesn't need to parse L3/L4 headers to implement this.
> > >
> > > My understanding from above is that HW, to be fully compatible with Linux, should produce CSUM
> > > starting from first byte after EtherType field:
> > >   (6 DST_MAC) (6 SRC_MAC) (2 EtherType) (...                   ...)
> > >                                          ^                       ^
> > >                                          | start csum            | end csum
> > > and ending at the last byte of Ethernet frame data.
> > > - if packet is VLAN tagged then VLAN TCI and real EtherType included in CSUM,
> > >    but first VLAN TPID doesn't
> > > - pad bytes may/may not be included in csum  
> 
> Based on commit 88078d98d1bb ("net: pskb_trim_rcsum() and
> CHECKSUM_COMPLETE are friends") these bytes are expected to be covered
> by skb->csum.
> 
> Not sure about that ipv4 header pull without csum adjust.

Isn't it just because IPv4 has a header checksum and therefore what's
pulled off adds up to 0 anyway? IPv6 does not have a header csum, hence
the adjustment?