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

Jesper Dangaard Brouer
2023-03-30T09:53:44Z

On 30/03/2023 01.19, Stanislav Fomichev wrote:
> On 03/29, Jesper Dangaard Brouer wrote:
>
>> On 29/03/2023 19.18, Stanislav Fomichev wrote:
>> > On 03/29, Jesper Dangaard Brouer wrote:
>> >
>> > > On 28/03/2023 23.58, Stanislav Fomichev wrote:
>> > > > On 03/28, Jesper Dangaard Brouer wrote:
>> > > > > 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.
>> > > >
>> > > > > 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.
>> > > >
>> > > > > 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?
>> >
>> > > 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).
>> >
>> > > We below layout you can:
>> >
>> > >   if (rss_type & XDP_RSS_TYPE_L4_MASK)
>> > >     bool hw_hash_do_LB = true;
>> >
>> > > Or using it as a number:
>> >
>> > >   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?
>> >
>> > > 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.
>> >
>> > > > 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),
>> >
>> > > 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.

See below, why I'm wrong (in storing this as numbers).

>> >
>> > [..]
>> >
>> > > 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.
>
> Agree with Toke on a separate thread; might not be too late to fit it
> into an rc..
>
>> > 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.
>
>> > > > }
>> > > >
>> > > > And then using XDP_RSS_TYPE_IPV4|XDP_RSS_TYPE_UDP vs
>> > > > XDP_RSS_TYPE_IPV6|XXX ?
>> >
>> > > 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.
>> >
>> >
>> > > 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.
>
> Yeah, I like it better. Btw, why have a separate bit for XDP_RSS_BIT_EX?

Yes, we can rename the EX bit define (which is in V2). I reduced the
name-length, because it allowed to keep code on-one-line when OR'ing.

> Any reason it's not a XDP_RSS_L3_IPV6_EX within XDP_RSS_L3_MASK?
>

Hmm... I guess it belongs with L3.

Do notice that both IPv4 and IPv6 have a flexible header called either
options/extensions headers, after their fixed header. (Mlx4 HW contains
this info for IPv4, but I didn't extend xdp_rss_hash_type in that patch).
Thus, we could have a single BIT that is valid for both IPv4 and IPv6.
(This can help speedup packet parsing having this info).

[...]
>
>> > 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.
>

The above code made me realize, I was wrong and you are right, we should
represent the L4 types as BITs (and not as numbers).
Even-though a single packet cannot be both UDP and TCP at the same time,
then it is reasonable to have a code path that want to match both UDP
and TCP. If L4 types are BITs then code can do a single compare (via
ORing), while if they are numbers then we need more compares.
Thus, I'll change scheme in V3 to use BITs.

>> > > > > 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?
>
> Let's fix the prototype while it's still early in the rc?

Okay, in V3 I will propose adding an argument for the type then.

> Maybe also extend the tests to drop/decode/verify the mask?

Yes, I/we obviously need to update the selftests.

One problem with selftests is that it's using veth SKB-based mode, and
SKB's have lost the RSS hash info and converted this into a single BIT
telling us if this was L4 based. Thus, its hard to do some e.g. UDP
type verification, but I guess we can check if expected UDP packet is
RSS type L4.

In xdp_hw_metadata, I will add something that uses the RSS type bits. I
was thinking to match against L4-UDP RSS type as program only AF_XDP
redirect UDP packets, so we can verify it was a UDP packet by HW info.

--Jesper