Re: [PATCH bpf-next v3 1/5] netlink: specs: Add XDP RX checksum capability to XDP metadata specs

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 18/02/2026 02.01, Stanislav Fomichev wrote:
> On 02/17, Lorenzo Bianconi wrote:
>> 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.
>>
>> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
>> ---
>>   Documentation/netlink/specs/netdev.yaml |  5 +++++
>>   include/net/xdp.h                       | 13 +++++++++++++
>>   include/uapi/linux/netdev.h             |  3 +++
>>   net/core/xdp.c                          | 28 ++++++++++++++++++++++++++++
>>   tools/include/uapi/linux/netdev.h       |  3 +++
>>   5 files changed, 52 insertions(+)
>>
>> diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
>> index 596c306ce52b8303b20680ff0cd34d4fd9db0e48..58eda634668a07860447a65d9fc2284839af6244 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 aa742f413c358575396530879af4570dc3fc18de..00abb2e1e85514b4080d0e4e6e3b8f5f67f73b61 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 +647,21 @@ enum xdp_rss_hash_type {
>>   	XDP_RSS_TYPE_L4_IPV6_SCTP_EX = XDP_RSS_TYPE_L4_IPV6_SCTP | XDP_RSS_L3_DYNHDR,
>>   };
>>   
>> +enum xdp_checksum {
>> +	XDP_CHECKSUM_NONE		= CHECKSUM_NONE,
>> +	XDP_CHECKSUM_UNNECESSARY	= CHECKSUM_UNNECESSARY,
>> +	XDP_CHECKSUM_COMPLETE		= CHECKSUM_COMPLETE,
>> +};
>> +
>>   struct xdp_metadata_ops {
>>   	int	(*xmo_rx_timestamp)(const struct xdp_md *ctx, u64 *timestamp);
>>   	int	(*xmo_rx_hash)(const struct xdp_md *ctx, u32 *hash,
>>   			       enum xdp_rss_hash_type *rss_type);
>>   	int	(*xmo_rx_vlan_tag)(const struct xdp_md *ctx, __be16 *vlan_proto,
>>   				   u16 *vlan_tci);
>> +	int	(*xmo_rx_checksum)(const struct xdp_md *ctx,
>> +				   enum xdp_checksum *ip_summed,
>> +				   u32 *cksum_meta);
>>   };
>>   
>>   #ifdef CONFIG_NET
>> diff --git a/include/uapi/linux/netdev.h b/include/uapi/linux/netdev.h
>> index e0b579a1df4f2126acec6c44c299e97bbbefe640..d20da430cfd57bc26b5ea2f406c27b48d8a81693 100644
>> --- a/include/uapi/linux/netdev.h
>> +++ b/include/uapi/linux/netdev.h
>> @@ -47,11 +47,14 @@ enum netdev_xdp_act {
>>    *   hash via bpf_xdp_metadata_rx_hash().
>>    * @NETDEV_XDP_RX_METADATA_VLAN_TAG: Device is capable of exposing receive
>>    *   packet VLAN tag via bpf_xdp_metadata_rx_vlan_tag().
>> + * @NETDEV_XDP_RX_METADATA_CHECKSUM: Device is capable of exposing receive
>> + *   checksum result via bpf_xdp_metadata_rx_checksum().
>>    */
>>   enum netdev_xdp_rx_metadata {
>>   	NETDEV_XDP_RX_METADATA_TIMESTAMP = 1,
>>   	NETDEV_XDP_RX_METADATA_HASH = 2,
>>   	NETDEV_XDP_RX_METADATA_VLAN_TAG = 4,
>> +	NETDEV_XDP_RX_METADATA_CHECKSUM = 8,
>>   };
>>   
>>   /**
>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>> index fee6d080ee85fc2d278bfdddfd1365633058ec06..7d1e08d8ab4151ab42c91203def2afafc66d3149 100644
>> --- a/net/core/xdp.c
>> +++ b/net/core/xdp.c
>> @@ -961,6 +961,34 @@ __bpf_kfunc int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
>>   	return -EOPNOTSUPP;
>>   }
>>   
>> +/**
>> + * bpf_xdp_metadata_rx_checksum - Read XDP frame RX checksum.
>> + * @ctx: XDP context pointer.
>> + * @ip_summed: Return value pointer indicating checksum result.
>> + * @cksum_meta: Return value pointer indicating checksum result metadata.
>> + *
>> + * In case of success, ``ip_summed`` is set to the RX checksum result. Possible
>> + * values are:
>> + * ``XDP_CHECKSUM_NONE``
>> + * ``XDP_CHECKSUM_UNNECESSARY``
>> + * ``XDP_CHECKSUM_COMPLETE``
>> + *
>> + * In case of success, ``cksum_meta`` contains the hw computed checksum value
>> + * for ``XDP_CHECKSUM_COMPLETE`` or the ``csum_level`` for
>> + * ``XDP_CHECKSUM_UNNECESSARY``. It is set to 0 for ``XDP_CHECKSUM_NONE``
> 
> The only thing I'm still not sure about is the csum_level and whether
> we need to export it or just start with csum_level=0 and extend later
> when needed. The rest looks good.
> 
> Jesper, Lorenzo mentioned that you might need it? Can you clarify?

At Cloudflare our load-balancer Unimog[1] does GUE (Generic UDP
Encapsulation) when XDP_TX'ing packets to neighboring servers.
Thus, I assume we want to know the csum_level, as this is for
encapsulated packets, right?

Cc Jesse, as he knows more about the hardware and csum_level. To Jesse,
we need to test how hardware handles our GUE packet format (which is
slightly modified).

Cc Arthur + Willem, as they knows the details around how Unimog
currently have to recalc packet checksums in software.  Hopefully this
patchset can help us avoid doing this in some cases.

--Jesper

  [1] 
https://blog.cloudflare.com/unimog-cloudflares-edge-load-balancer/#encapsulation
  [Patch-0/5] 
https://lore.kernel.org/all/20260217-bpf-xdp-meta-rxcksum-v3-0-30024c50ba71@kernel.org/