[xdp-hints] Re: [PATCH bpf-next v3 12/21] xdp: Add checksum hint

["Zaremba, Larysa" <larysa.zaremba@intel.com>]

On Thu, Jul 20, 2023 at 09:57:05AM +0000, Zaremba, Larysa wrote:
> On Wed, Jul 19, 2023 at 05:42:04PM -0400, Willem de Bruijn wrote:
> > Larysa Zaremba wrote:
> > > Implement functionality that enables drivers to expose to XDP code checksum
> > > information that consists of:
> > > 
> > > - Checksum status - bitfield that consists of
> > >   - number of consecutive validated checksums. This is almost the same as
> > >     csum_level in skb, but starts with 1. Enum names for those bits still
> > >     use checksum level concept, so it is less confusing for driver
> > >     developers.
> > >   - Is checksum partial? This bit cannot coexist with any other
> > >   - Is there a complete checksum available?
> > > - Additional checksum data, a union of:
> > >   - checksum start and offset, if checksum is partial
> > >   - complete checksum, if available
> > > 
> > > Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> > > ---
> > >  Documentation/networking/xdp-rx-metadata.rst |  3 ++
> > >  include/linux/netdevice.h                    |  3 ++
> > >  include/net/xdp.h                            | 46 ++++++++++++++++++++
> > >  kernel/bpf/offload.c                         |  2 +
> > >  net/core/xdp.c                               | 23 ++++++++++
> > >  5 files changed, 77 insertions(+)
> > > 
> > > diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst
> > > index ea6dd79a21d3..7f056a44f682 100644
> > > --- a/Documentation/networking/xdp-rx-metadata.rst
> > > +++ b/Documentation/networking/xdp-rx-metadata.rst
> > > @@ -26,6 +26,9 @@ metadata is supported, this set will grow:
> > >  .. kernel-doc:: net/core/xdp.c
> > >     :identifiers: bpf_xdp_metadata_rx_vlan_tag
> > >  
> > > +.. kernel-doc:: net/core/xdp.c
> > > +   :identifiers: bpf_xdp_metadata_rx_csum
> > > +
> > >  An XDP program can use these kfuncs to read the metadata into stack
> > >  variables for its own consumption. Or, to pass the metadata on to other
> > >  consumers, an XDP program can store it into the metadata area carried
> > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > index 1749f4f75c64..4f6da36ac123 100644
> > > --- a/include/linux/netdevice.h
> > > +++ b/include/linux/netdevice.h
> > > @@ -1660,6 +1660,9 @@ struct xdp_metadata_ops {
> > >  			       enum xdp_rss_hash_type *rss_type);
> > >  	int	(*xmo_rx_vlan_tag)(const struct xdp_md *ctx, u16 *vlan_tci,
> > >  				   __be16 *vlan_proto);
> > > +	int	(*xmo_rx_csum)(const struct xdp_md *ctx,
> > > +			       enum xdp_csum_status *csum_status,
> > > +			       union xdp_csum_info *csum_info);
> > >  };
> > >  
> > >  /**
> > > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > > index 89c58f56ffc6..2b7a7d678ff4 100644
> > > --- a/include/net/xdp.h
> > > +++ b/include/net/xdp.h
> > > @@ -391,6 +391,8 @@ void xdp_attachment_setup(struct xdp_attachment_info *info,
> > >  			   bpf_xdp_metadata_rx_hash) \
> > >  	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_VLAN_TAG, \
> > >  			   bpf_xdp_metadata_rx_vlan_tag) \
> > > +	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_CSUM, \
> > > +			   bpf_xdp_metadata_rx_csum) \
> > >  
> > >  enum {
> > >  #define XDP_METADATA_KFUNC(name, _) name,
> > > @@ -448,6 +450,50 @@ enum xdp_rss_hash_type {
> > >  	XDP_RSS_TYPE_L4_IPV6_SCTP_EX = XDP_RSS_TYPE_L4_IPV6_SCTP | XDP_RSS_L3_DYNHDR,
> > >  };
> > >  
> > > +union xdp_csum_info {
> > > +	/* Checksum referred to by ``csum_start + csum_offset`` is considered
> > > +	 * valid, but was never calculated, TX device has to do this,
> > > +	 * starting from csum_start packet byte.
> > > +	 * Any preceding checksums are also considered valid.
> > > +	 * Available, if ``status == XDP_CHECKSUM_PARTIAL``.
> > > +	 */
> > > +	struct {
> > > +		u16 csum_start;
> > > +		u16 csum_offset;
> > > +	};
> > > +
> > > +	/* Checksum, calculated over the whole packet.
> > > +	 * Available, if ``status & XDP_CHECKSUM_COMPLETE``.
> > > +	 */
> > > +	u32 checksum;
> > > +};
> > > +
> > > +enum xdp_csum_status {
> > > +	/* HW had parsed several transport headers and validated their
> > > +	 * checksums, same as ``CHECKSUM_UNNECESSARY`` in ``sk_buff``.
> > > +	 * 3 least significat bytes contain number of consecutive checksum,
> > 
> > typo: significant
> > 
> > (I did not scan for typos, just came across this when trying to understand
> > the skb->csum_level + 1 trick. Probably good to run a spell check).
> >

Oh, and about skb->csum_level + 1, maybe this way it would be more 
understandable: XDP_CHECKSUM_VALID_LVL0 + skb->csum_level?

Using number of valid checksums (starts with 1) instead of checksum level 
(starts with 0) is a debatable decision, but I have decided to go with it under 
2 assumptions:

- the only reason checksum level in skb starts with 0 is to use less bits
- checksum number would be more intuitive for XDP/AF_XDP application developers

I encourage everyone to share their opinion on that.
 
> 
> Thanks! Will fix.
> 
> > > +	 * starting with the outermost, reported by hardware as valid.
> > > +	 * ``sk_buff`` checksum level (``csum_level``) notation is provided
> > > +	 * for driver developers.
> > > +	 */
> > > +	XDP_CHECKSUM_VALID_LVL0		= 1,	/* 1 outermost checksum */
> > > +	XDP_CHECKSUM_VALID_LVL1		= 2,	/* 2 outermost checksums */
> > > +	XDP_CHECKSUM_VALID_LVL2		= 3,	/* 3 outermost checksums */
> > > +	XDP_CHECKSUM_VALID_LVL3		= 4,	/* 4 outermost checksums */
> > > +	XDP_CHECKSUM_VALID_NUM_MASK	= GENMASK(2, 0),
> > > +	XDP_CHECKSUM_VALID		= XDP_CHECKSUM_VALID_NUM_MASK,
> > > +
> > > +	/* Occurs if packet is sent virtually (between Linux VMs / containers)
> > > +	 * This status cannot coexist with any other.
> > > +	 * Refer to ``csum_start`` and ``csum_offset`` in ``xdp_csum_info``
> > > +	 * for more information.
> > > +	 */
> > > +	XDP_CHECKSUM_PARTIAL	= BIT(3),
> > > +
> > > +	/* Checksum, calculated over the entire packet is provided */
> > > +	XDP_CHECKSUM_COMPLETE	= BIT(4),
> > > +};
> > > +
> > >  #ifdef CONFIG_NET
> > >  u32 bpf_xdp_metadata_kfunc_id(int id);
> > >  bool bpf_dev_bound_kfunc_id(u32 btf_id);
> > > diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
> > > index 986e7becfd42..f60a6add5273 100644
> > > --- a/kernel/bpf/offload.c
> > > +++ b/kernel/bpf/offload.c
> > > @@ -850,6 +850,8 @@ void *bpf_dev_bound_resolve_kfunc(struct bpf_prog *prog, u32 func_id)
> > >  		p = ops->xmo_rx_hash;
> > >  	else if (func_id == bpf_xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_VLAN_TAG))
> > >  		p = ops->xmo_rx_vlan_tag;
> > > +	else if (func_id == bpf_xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_CSUM))
> > > +		p = ops->xmo_rx_csum;
> > >  out:
> > >  	up_read(&bpf_devs_lock);
> > >  
> > > diff --git a/net/core/xdp.c b/net/core/xdp.c
> > > index 8b55419d332e..d4ea54046afc 100644
> > > --- a/net/core/xdp.c
> > > +++ b/net/core/xdp.c
> > > @@ -772,6 +772,29 @@ __bpf_kfunc int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
> > >  	return -EOPNOTSUPP;
> > >  }
> > >  
> > > +/**
> > > + * bpf_xdp_metadata_rx_csum - Get checksum status with additional info.
> > > + * @ctx: XDP context pointer.
> > > + * @csum_status: Destination for checksum status.
> > > + * @csum_info: Destination for complete checksum or partial checksum offset.
> > > + *
> > > + * Status (@csum_status) is a bitfield that informs, what checksum
> > > + * processing was performed. Additional results of such processing,
> > > + * such as complete checksum or partial checksum offsets,
> > > + * are passed as info (@csum_info).
> > > + *
> > > + * Return:
> > > + * * Returns 0 on success or ``-errno`` on error.
> > > + * * ``-EOPNOTSUPP`` : device driver doesn't implement kfunc
> > > + * * ``-ENODATA``    : Checksum status is unknown
> > > + */
> > > +__bpf_kfunc int bpf_xdp_metadata_rx_csum(const struct xdp_md *ctx,
> > > +					 enum xdp_csum_status *csum_status,
> > > +					 union xdp_csum_info *csum_info)
> > > +{
> > > +	return -EOPNOTSUPP;
> > > +}
> > > +
> > >  __diag_pop();
> > >  
> > >  BTF_SET8_START(xdp_metadata_kfunc_ids)
> > > -- 
> > > 2.41.0
> > > 
> > 
> > 
>