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

[Larysa Zaremba <larysa.zaremba@intel.com>]

On Sun, Jul 30, 2023 at 09:13:02AM -0400, Willem de Bruijn wrote:
> 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?
>

+1
CHECKSUM_PARTIAL is mostly for testing and removing/adding it doesn't change 
anything from the perspective of the user that does not use it, so I think it is 
worth having.

> > > > > +   /* 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.

Please look into csum_level field in sk_buff. It is not the most used property 
in the kernel networking code, but it is certainly 1. used by networking stack 
2. set to non-zero value by many vendors.

So you do not need to search yourself, I'll copy-paste the docs for 
CHECKSUM_UNNECESSARY here:

 *   %CHECKSUM_UNNECESSARY is applicable to following protocols:
 *
 *     - TCP: IPv6 and IPv4.
 *     - UDP: IPv4 and IPv6. A device may apply CHECKSUM_UNNECESSARY to a
 *       zero UDP checksum for either IPv4 or IPv6, the networking stack
 *       may perform further validation in this case.
 *     - GRE: only if the checksum is present in the header.
 *     - SCTP: indicates the CRC in SCTP header has been validated.
 *     - FCOE: indicates the CRC in FC frame has been validated.
 *

Please, look at this:

 *   &sk_buff.csum_level indicates the number of consecutive checksums found in
 *   the packet minus one that have been verified as %CHECKSUM_UNNECESSARY.
 *   For instance if a device receives an IPv6->UDP->GRE->IPv4->TCP packet
 *   and a device is able to verify the checksums for UDP (possibly zero),
 *   GRE (checksum flag is set) and TCP, &sk_buff.csum_level would be set to
 *   two. If the device were only able to verify the UDP checksum and not
 *   GRE, either because it doesn't support GRE checksum or because GRE
 *   checksum is bad, skb->csum_level would be set to zero (TCP checksum is
 *   not considered in this case).

From: 
https://elixir.bootlin.com/linux/v6.5-rc4/source/include/linux/skbuff.h#L115

> > The driver should say CHECKSUM_UNNECESSARY when it's sure
> > or don't pretend that it checks the checksum and just say NONE.
> 

Well, in such case, most of the NICs that use CHECKSUM_UNNECESSARY would have to 
return CHECKSUM_NONE instead, because based on my quick search, they mostly 
return checksum level of 0 (no tunneling detected) or 1 (tunneling detected),
so they only parse headers up to a certain depth, meaning it's not possible
to tell whether there isn't another CHECKSUM_UNNECESSARY-eligible header hiding
in the payload, so those NIC cannot guarantee ALL the checksums present in the 
packet are correct. So, by your logic, we should make e.g. AF_XDP user re-check 
already verified checksums themselves, because HW "doesn't pretend that it 
checks the checksum and just says 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.

Yes, there are several vendors that set the csum_level, including broadcom 
(bnxt) and mellanox (mlx4 and mlx5).

Also, CHECKSUM_UNNECESSARY is found in 100+ drivers/net/ethernet files,
while csum_level is in like 20, which means overwhelming majority of 
CHECKSUM_UNNECESSARY NICs actually stay with the default checksum level of '0'
(they check only the outermost checksum - anything else needs to be verified by 
the networking stack).