Re: [PATCH bpf RFC 1/4] xdp: rss hash types representation

On 29/03/2023 19.18, Stanislav Fomichev wrote:

On 03/29, Jesper Dangaard Brouer wrote:

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On 28/03/2023 23.58, Stanislav Fomichev wrote:

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On 03/28, Jesper Dangaard Brouer wrote:

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The RSS hash type specifies what portion of packet data NIC hardware used
when calculating RSS hash value. The RSS types are focused on Internet
traffic protocols at OSI layers L3 and L4. L2 (e.g. ARP) often get hash
value zero and no RSS type. For L3 focused on IPv4 vs. IPv6, and L4
primarily TCP vs UDP, but some hardware supports SCTP.

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Hardware RSS types are differently encoded for each hardware NIC. Most
hardware represent RSS hash type as a number. Determining L3 vs L4 often
requires a mapping table as there often isn't a pattern or sorting
according to ISO layer.

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The patch introduce a XDP RSS hash type (xdp_rss_hash_type) that can both
be seen as a number that is ordered according by ISO layer, and can be bit
masked to separate IPv4 and IPv6 types for L4 protocols. Room is available
for extending later while keeping these properties. This maps and unifies
difference to hardware specific hashes.

Looks good overall. Any reason we're making this specific layout?

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One important goal is to have a simple/fast way to determining L3 vs L4,
because a L4 hash can be used for flow handling (e.g. load-balancing).

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We below layout you can:

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  if (rss_type & XDP_RSS_TYPE_L4_MASK)
    bool hw_hash_do_LB = true;

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Or using it as a number:

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  if (rss_type > XDP_RSS_TYPE_L4)
    bool hw_hash_do_LB = true;

Why is it strictly better then the following?

if (rss_type & (TYPE_UDP | TYPE_TCP | TYPE_SCTP)) {}

See V2 I dropped the idea of this being a number (that idea was not a
good idea).

If we add some new L4 format, the bpf programs can be updated to support
it?

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I'm very open to changes to my "specific" layout.  I am in doubt if
using it as a number is the right approach and worth the trouble.

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Why not simply the following?

enum {
 ����XDP_RSS_TYPE_NONE = 0,
 ����XDP_RSS_TYPE_IPV4 = BIT(0),
 ����XDP_RSS_TYPE_IPV6 = BIT(1),
 ����/* IPv6 with extension header. */
 ����/* let's note ^^^ it in the UAPI? */
 ����XDP_RSS_TYPE_IPV6_EX = BIT(2),
 ����XDP_RSS_TYPE_UDP = BIT(3),
 ����XDP_RSS_TYPE_TCP = BIT(4),
 ����XDP_RSS_TYPE_SCTP = BIT(5),

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We know these bits for UDP, TCP, SCTP (and IPSEC) are exclusive, they
cannot be set at the same time, e.g. as a packet cannot both be UDP and
TCP.  Thus, using these bits as a number make sense to me, and is more
compact.

[..]

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This BIT() approach also have the issue of extending it later (forward
compatibility).  As mentioned a common task will be to check if
hash-type is a L4 type.  See mlx5 [patch 4/4] needed to extend with
IPSEC. Notice how my XDP_RSS_TYPE_L4_MASK covers all the bits that this
can be extended with new L4 types, such that existing progs will still
work checking for L4 check.  It can of-cause be solved in the same way
for this BIT() approach by reserving some bits upfront in a mask.

We're using 6 bits out of 64, we should be good for awhile? If there
is ever a forward compatibility issue, we can always come up with
a new kfunc.

I want/need store the RSS-type in the xdp_frame, for XDP_REDIRECT and
SKB use-cases.  Thus, I don't want to use 64-bit/8-bytes, as xdp_frame
size is limited (given it reduces headroom expansion).

One other related question I have is: should we export the type
over some additional new kfunc argument? (instead of abusing the return
type) 

Good question. I was also wondering if it wouldn't be better to add
another kfunc argument with the rss_hash_type?

That will change the call signature, so that will not be easy to handle
between kernel releases.

Maybe that will let us drop the explicit BTF_TYPE_EMIT as well?

Sure, if we define it as an argument, then it will automatically
exported as BTF.

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}

And then using XDP_RSS_TYPE_IPV4|XDP_RSS_TYPE_UDP vs
XDP_RSS_TYPE_IPV6|XXX ?

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Do notice, that I already does some level of or'ing ("|") in this
proposal.  The main difference is that I hide this from the driver, and
kind of pre-combine the valid combination (enum's) drivers can select
from. I do get the point, and I think I will come up with a combined
solution based on your input.

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The RSS hashing types and combinations comes from M$ standards:
  [1] 
https://learn.microsoft.com/en-us/windows-hardware/drivers/network/rss-hashing-types#ipv4-hash-type-combinations

My main concern here is that we're over-complicating it with the masks
and the format. With the explicit bits we can easily map to that
spec you mention.

See if you like my RFC-V2 proposal better.
It should go more in your direction.

For example, for forward compat, I'm not sure we can assume that the people
will do:
     "rss_type & XDP_RSS_TYPE_L4_MASK"
instead of something like:
     "rss_type & (XDP_RSS_TYPE_L4_IPV4_TCP|XDP_RSS_TYPE_L4_IPV4_UDP)"

This code is allowed in V2 and should be. It is a choice of
BPF-programmer in line-2 to not be forward compatible with newer L4 types.

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This proposal change the kfunc API bpf_xdp_metadata_rx_hash() to  return
this RSS hash type on success.

This is the real question (as also raised above)...
Should we use return value or add an argument for type?

--Jesper