Re: [PATCH bpf-next v4 1/6] netlink: specs: Add XDP RX checksum capability to XDP metadata specs

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On 07/08, Vladimir Vdovin wrote:
> > From: Lorenzo Bianconi <lorenzo@kernel.org>
> > 
> > Introduce XDP RX checksum capability to XDP metadata specs. XDP RX
> > checksum will be use by devices capable of exposing receive checksum
> > result via bpf_xdp_metadata_rx_checksum().
> > Moreover, introduce xmo_rx_checksum netdev callback in order to allow
> > the eBPF program bound to the device to retrieve the RX checksum result
> > computed by the hw NIC and reported via DMA descriptors.
> > 
> > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > Signed-off-by: Vladimir Vdovin <deliran@verdict.gg>
> > ---
> >  Documentation/netlink/specs/netdev.yaml |  5 ++++
> >  include/net/xdp.h                       | 18 ++++++++++++++
> >  include/uapi/linux/netdev.h             |  3 +++
> >  net/core/xdp.c                          | 32 +++++++++++++++++++++++++
> >  tools/include/uapi/linux/netdev.h       |  3 +++
> >  5 files changed, 61 insertions(+)
> > 
> > diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> > index 5f143da7458c..6d0d90d3a614 100644
> > --- a/Documentation/netlink/specs/netdev.yaml
> > +++ b/Documentation/netlink/specs/netdev.yaml
> > @@ -61,6 +61,11 @@ definitions:
> >          doc: |
> >            Device is capable of exposing receive packet VLAN tag via
> >            bpf_xdp_metadata_rx_vlan_tag().
> > +      -
> > +        name: checksum
> > +        doc: |
> > +          Device is capable of exposing receive checksum result via
> > +          bpf_xdp_metadata_rx_checksum().
> >    -
> >      type: flags
> >      name: xsk-flags
> > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > index aa742f413c35..e255ff786131 100644
> > --- a/include/net/xdp.h
> > +++ b/include/net/xdp.h
> > @@ -586,6 +586,10 @@ void xdp_attachment_setup(struct xdp_attachment_info *info,
> >  			   NETDEV_XDP_RX_METADATA_VLAN_TAG, \
> >  			   bpf_xdp_metadata_rx_vlan_tag, \
> >  			   xmo_rx_vlan_tag) \
> > +	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_CHECKSUM, \
> > +			   NETDEV_XDP_RX_METADATA_CHECKSUM, \
> > +			   bpf_xdp_metadata_rx_checksum, \
> > +			   xmo_rx_checksum)
> >  
> >  enum xdp_rx_metadata {
> >  #define XDP_METADATA_KFUNC(name, _, __, ___) name,
> > @@ -643,12 +647,26 @@ enum xdp_rss_hash_type {
> >  	XDP_RSS_TYPE_L4_IPV6_SCTP_EX = XDP_RSS_TYPE_L4_IPV6_SCTP | XDP_RSS_L3_DYNHDR,
> >  };
> 
> [..]
>  
> > +/* Please note the driver is required to invalidate the checksum if the NIC
> > + * reports CHECKSUM_UNNECESSARY or CHECKSUM_COMPLETE and the eBPF program
> > + * modifies the packet since it can change some fields validated by the
> > + * checksum.
> > + */
> 
> Sorry, a bit confused about this part. IIUC this is in response to this
> discussion https://lore.kernel.org/bpf/aaLYfWnuuf_ne72u@lore-desk/ ?
> 
> Looking at a few drivers:
> - bnxt (bnxt_rx_pkt) does UNNECESSARY - ok
> - mlx5 (mlx5e_handle_csum) does UNNECESSARY and skips COMPLETE if there is
>   bpf prog attached
> - fbnic (fbnic_rx_csum) - can do COMPLETE even with xdp attached?
> - gve (gve_rx) - can do COMPLETE even with xdp attached?
> 
> So none of the drivers invalidate anything (and nether the bpf machinery), some
> choose to (correctly?) not report COMPLETE when there is XDP. So was the
> discussion about documenting this and giving some guidance? This probably
> belongs to that `DOC: skb checksums` section of include/linux/skbuff.h?

Hi Stanislav,

re-reading the previous upstream discussion, I guess we should just document the
expected driver behaviour (invalidate the cksum for both COMPLETE and
UNNECESSARY cases when the XDP program running on the NIC returns XDP_PASS) and
add a given kselftest for it. Agree?

> 
> Also, can you add your new rx_checksum to
> Documentation/networking/xdp-rx-metadata.rst's list of supported kfuncs?

ack, I will do it in the next iteration.

Regards,
Lorenzo

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