netdev - Re: [PATCH bpf-next v4 12/21] xdp: Add checksum hint

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Message-ID: <64c53b1b29a66_e235c2942d@willemb.c.googlers.com.notmuch>
Date: Sat, 29 Jul 2023 12:15:23 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>, 
 Larysa Zaremba <larysa.zaremba@...el.com>
Cc: bpf@...r.kernel.org, 
 ast@...nel.org, 
 daniel@...earbox.net, 
 andrii@...nel.org, 
 martin.lau@...ux.dev, 
 song@...nel.org, 
 yhs@...com, 
 john.fastabend@...il.com, 
 kpsingh@...nel.org, 
 sdf@...gle.com, 
 haoluo@...gle.com, 
 jolsa@...nel.org, 
 David Ahern <dsahern@...il.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Willem de Bruijn <willemb@...gle.com>, 
 Jesper Dangaard Brouer <brouer@...hat.com>, 
 Anatoly Burakov <anatoly.burakov@...el.com>, 
 Alexander Lobakin <alexandr.lobakin@...el.com>, 
 Magnus Karlsson <magnus.karlsson@...il.com>, 
 Maryam Tahhan <mtahhan@...hat.com>, 
 xdp-hints@...-project.net, 
 netdev@...r.kernel.org, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Simon Horman <simon.horman@...igine.com>
Subject: Re: [PATCH bpf-next v4 12/21] xdp: Add checksum hint

Alexei Starovoitov wrote:
> On Fri, Jul 28, 2023 at 07:39:14PM +0200, Larysa Zaremba wrote:
> >  
> > +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_PARTIAL makes sense on TX, but this RX. I don't see in the above.

It can be observed on RX when packets are looped.

This may be observed even in XDP on veth.
 
> > +	/* Checksum, calculated over the whole packet.
> > +	 * Available, if ``status & XDP_CHECKSUM_COMPLETE``.
> > +	 */
> > +	u32 checksum;
> 
> imo XDP RX should only support XDP_CHECKSUM_COMPLETE with u32 checksum
> or XDP_CHECKSUM_UNNECESSARY.
> 
> > +};
> > +
> > +enum xdp_csum_status {
> > +	/* HW had parsed several transport headers and validated their
> > +	 * checksums, same as ``CHECKSUM_UNNECESSARY`` in ``sk_buff``.
> > +	 * 3 least significant bytes contain number of consecutive checksums,
> > +	 * 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,
> 
> I don't see what bpf prog suppose to do with these levels.
> The driver should pick between 3:
> XDP_CHECKSUM_UNNECESSARY, XDP_CHECKSUM_COMPLETE, XDP_CHECKSUM_NONE.
> 
> No levels and no anything partial. please.

This levels business is an unfortunate side effect of
CHECKSUM_UNNECESSARY. For a packet with multiple checksum fields, what
does the boolean actually mean? With these levels, at least that is
well defined: the first N checksum fields.