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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Alexei Starovoitov wrote:
> On Sat, Jul 29, 2023 at 9:15 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > 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.
> 
> veth and XDP is a broken combination. GSO packets coming out of containers
> cannot be parsed properly by XDP.
> It was added mainly for testing. Just like "generic XDP".
> bpf progs at skb layer is much better fit for veth.

Ok. Still, seems forward looking and little cost to define the
constant?
 
> > > > +   /* 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.
> 
> If I understand this correctly this is intel specific feature that
> other NICs don't have. skb layer also doesn't have such concept.
> The driver should say CHECKSUM_UNNECESSARY when it's sure
> or don't pretend that it checks the checksum and just say NONE.

I did not know how much this was used, but quick grep for non constant
csum_level shows devices from at least six vendors.