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

From: Larysa Zaremba <[email-masked]>
Date: Tue, 4 Jul 2023 13:19:24 +0200

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 <[email-masked]>
> > > > ---
> > > >   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.
 
> > > > +{
> > > > +	return -EOPNOTSUPP;
> > > > +}
> > > > +
> > > >   __diag_pop();