netdev - Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow path

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Date:   Mon, 29 Mar 2021 17:00:09 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alexander Lobakin <alobakin@...me>
Subject: Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow
 path

On Mon, 2021-03-29 at 09:52 -0400, Willem de Bruijn wrote:
> > > That breaks the checksum-and-copy optimization when delivering to
> > > local sockets. I wonder if that is a regression.
> > 
> > The conversion to CHECKSUM_UNNECESSARY happens since
> > commit 573e8fca255a27e3573b51f9b183d62641c47a3d.
> > 
> > Even the conversion to CHECKSUM_PARTIAL happens independently from this
> > series, since commit 6f1c0ea133a6e4a193a7b285efe209664caeea43.
> > 
> > I don't see a regression here ?!?
> 
> I mean that UDP packets with local destination socket and no tunnels
> that arrive with CHECKSUM_NONE normally benefit from the
> checksum-and-copy optimization in recvmsg() when copying to user.
> 
> If those packets are now checksummed during GRO, that voids that
> optimization, and the packet payload is now touched twice.

The 'now' part confuses me. Nothing in this patch or this series
changes the processing of CHECKSUM_NONE UDP packets with no tunnel.

I do see checksum validation in the GRO engine for CHECKSUM_NONE UDP
packet prior to this series.

I *think* the checksum-and-copy optimization is lost
since 573e8fca255a27e3573b51f9b183d62641c47a3d.

Regards,

Paolo