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@xxxxxxxxx> 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).
