[xdp-hints] Re: [PATCH bpf-next v2 12/20] xdp: Add checksum level hint

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 06/07/2023 07.50, John Fastabend wrote:
> Larysa Zaremba wrote:
>> On Tue, Jul 04, 2023 at 12:39:06PM +0200, Jesper Dangaard Brouer wrote:
>>> Cc. DaveM+Alex Duyck, as I value your insights on checksums.
>>>
>>> On 04/07/2023 11.24, Larysa Zaremba wrote:
>>>> On Mon, Jul 03, 2023 at 01:38:27PM -0700, John Fastabend wrote:
>>>>> Larysa Zaremba wrote:
>>>>>> Implement functionality that enables drivers to expose to XDP code,
>>>>>> whether checksums was checked and on what level.
>>>>>>
>>>>>> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
>>>>>> ---
>>>>>>    Documentation/networking/xdp-rx-metadata.rst |  3 +++
>>>>>>    include/linux/netdevice.h                    |  1 +
>>>>>>    include/net/xdp.h                            |  2 ++
>>>>>>    kernel/bpf/offload.c                         |  2 ++
>>>>>>    net/core/xdp.c                               | 21 ++++++++++++++++++++
>>>>>>    5 files changed, 29 insertions(+)
>>>>>>
>>>>>> diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst
>>>>>> index ea6dd79a21d3..4ec6ddfd2a52 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_lvl
>>>>>> +
>>>>>>    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 4fa4380e6d89..569563687172 100644
>>>>>> --- a/include/linux/netdevice.h
>>>>>> +++ b/include/linux/netdevice.h
>>>>>> @@ -1660,6 +1660,7 @@ struct xdp_metadata_ops {
>>>>>>    			       enum xdp_rss_hash_type *rss_type);
>>>>>>    	int	(*xmo_rx_vlan_tag)(const struct xdp_md *ctx, u16 *vlan_tag,
>>>>>>    				   __be16 *vlan_proto);
>>>>>> +	int	(*xmo_rx_csum_lvl)(const struct xdp_md *ctx, u8 *csum_level);
>>>>>>    };
>>>>>>    /**
>>>>>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>>>>>> index 89c58f56ffc6..61ed38fa79d1 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_LVL, \
>>>>>> +			   bpf_xdp_metadata_rx_csum_lvl) \
>>>>>>    enum {
>>>>>>    #define XDP_METADATA_KFUNC(name, _) name,
>>>>>> diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
>>>>>> index 986e7becfd42..a133fb775f49 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_LVL))
>>>>>> +		p = ops->xmo_rx_csum_lvl;
>>>>>>    out:
>>>>>>    	up_read(&bpf_devs_lock);
>>>>>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>>>>>> index f6262c90e45f..c666d3e0a26c 100644
>>>>>> --- a/net/core/xdp.c
>>>>>> +++ b/net/core/xdp.c
>>>>>> @@ -758,6 +758,27 @@ __bpf_kfunc int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx, u16 *vlan
>>>>>>    	return -EOPNOTSUPP;
>>>>>>    }
>>>>>> +/**
>>>>>> + * bpf_xdp_metadata_rx_csum_lvl - Get depth at which HW has checked the checksum.
>>>>>> + * @ctx: XDP context pointer.
>>>>>> + * @csum_level: Return value pointer.
>>>>>> + *
>>>>>> + * In case of success, csum_level contains depth of the last verified checksum.
>>>>>> + * If only the outermost checksum was verified, csum_level is 0, if both
>>>>>> + * encapsulation and inner transport checksums were verified, csum_level is 1,
>>>>>> + * and so on.
>>>>>> + * For more details, refer to csum_level field in sk_buff.
>>>>>> + *
>>>>>> + * Return:
>>>>>> + * * Returns 0 on success or ``-errno`` on error.
>>>>>> + * * ``-EOPNOTSUPP`` : device driver doesn't implement kfunc
>>>>>> + * * ``-ENODATA``    : Checksum was not validated
>>>>>> + */
>>>>>> +__bpf_kfunc int bpf_xdp_metadata_rx_csum_lvl(const struct xdp_md *ctx, u8 *csum_level)
>>>>>
>>>>> Istead of ENODATA should we return what would be put in the ip_summed field
>>>>> CHECKSUM_{NONE, UNNECESSARY, COMPLETE, PARTIAL}? Then sig would be,
>>>
>>> I was thinking the same, what about checksum "type".
>>>
>>>>>
>>>>>    bpf_xdp_metadata_rx_csum_lvl(const struct xdp_md *ctx, u8 *type, u8 *lvl);
>>>>>
>>>>> or something like that? Or is the thought that its not really necessary?
>>>>> I don't have a strong preference but figured it was worth asking.
>>>>>
>>>>
>>>> I see no value in returning CHECKSUM_COMPLETE without the actual checksum value.
>>>> Same with CHECKSUM_PARTIAL and csum_start. Returning those values too would
>>>> overcomplicate the function signature.
>>>
>>> So, this kfunc bpf_xdp_metadata_rx_csum_lvl() success is it equivilent to
>>> CHECKSUM_UNNECESSARY?
>>
>> This is 100% true for physical NICs, it's more complicated for veth, bacause it
>> often receives CHECKSUM_PARTIAL, which shouldn't normally apprear on RX, but is
>> treated by the network stack as a validated checksum, because there is no way
>> internally generated packet could be messed up. I would be grateful if you could
>> look at the veth patch and share your opinion about this.
>>
>>>
>>> Looking at documentation[1] (generated from skbuff.h):
>>>   [1] https://kernel.org/doc/html/latest/networking/skbuff.html#checksumming-of-received-packets-by-device
>>>
>>> Is the idea that we can add another kfunc (new signature) than can deal
>>> with the other types of checksums (in a later kernel release)?
>>>
>>
>> Yes, that is the idea.
> 
> If we think there is a chance we might need another kfunc we should add it
> in the same kfunc. It would be unfortunate to have to do two kfuncs when
> one would work. It shouldn't cost much/anything(?) to hardcode the type for
> most cases? I think if we need it later I would advocate for updating this
> kfunc to support it. Of course then userspace will have to swivel on the
> kfunc signature.
> 

I think it might make sense to have 3 kfuncs for checksumming.
As this would allow BPF-prog to focus on CHECKSUM_UNNECESSARY, and then
only call additional kfunc for extracting e.g csum_start  + csum_offset
when type is CHECKSUM_PARTIAL.

We could extend bpf_xdp_metadata_rx_csum_lvl() to give the csum_type
CHECKSUM_{NONE, UNNECESSARY, COMPLETE, PARTIAL}.

  int bpf_xdp_metadata_rx_csum_lvl(*ctx, u8 *csum_level, u8 *csum_type)

And then add two kfunc e.g.
  (1) bpf_xdp_metadata_rx_csum_partial(ctx, start, offset)
  (2) bpf_xdp_metadata_rx_csum_complete(ctx, csum)

Pseudo BPF-prog code:

  err = bpf_xdp_metadata_rx_csum_lvl(ctx, level, type);
  if (!err && type != CHECKSUM_UNNECESSARY) {
      if (type == CHECKSUM_PARTIAL)
          err = bpf_xdp_metadata_rx_csum_partial(ctx, start, offset);
      if (type == CHECKSUM_COMPLETE)
          err = bpf_xdp_metadata_rx_csum_complete(ctx, csum);
  }

Looking at code, I feel we could rename [...]_csum_lvl to csum_type.
E.g. bpf_xdp_metadata_rx_csum_type.

Feel free to disagree,
--Jesper