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

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Date:   Thu, 28 Jan 2021 15:00:12 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Xin Long <lucien.xin@...il.com>,
        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>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCHv3 net-next 1/2] net: support ip generic csum processing in skb_csum_hwoffload_help

On Thu, Jan 28, 2021 at 2:46 PM Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> On Thu, Jan 28, 2021 at 6:07 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Thu, Jan 28, 2021 at 4:29 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.
> > >
> > > 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 <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)) {
> >
> > 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.

I understand. But letting through an IPv6 packet to a nic that
advertises NETIF_F_IP_CSUM, but not NETIF_F_IPV6_CSUM, is still
incorrect, right?