Subject: Re: [xdp-hints] Re: [RFC bpf-next 0/5] xdp: hints via kfuncs

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 03 Nov 2022 13:48:59 +0100

Jesper Dangaard Brouer <[email-masked]> writes:

> On 03/11/2022 01.09, Toke Høiland-Jørgensen wrote:
>> Stanislav Fomichev <[email-masked]> writes:
>> 
>>> On Wed, Nov 2, 2022 at 3:02 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>>>
>>>> Jesper Dangaard Brouer <[email-masked]> writes:
>>>>
>>>>> On 01/11/2022 18.05, Martin KaFai Lau wrote:
>>>>>> On 10/31/22 6:59 PM, Stanislav Fomichev wrote:
>>>>>>> On Mon, Oct 31, 2022 at 3:57 PM Martin KaFai Lau
>>>>>>> <[email-masked]> wrote:
>>>>>>>>
>>>>>>>> On 10/31/22 10:00 AM, Stanislav Fomichev wrote:
>>>>>>>>>> 2. AF_XDP programs won't be able to access the metadata without
>>>>>>>>>> using a
>>>>>>>>>> custom XDP program that calls the kfuncs and puts the data into the
>>>>>>>>>> metadata area. We could solve this with some code in libxdp,
>>>>>>>>>> though; if
>>>>>>>>>> this code can be made generic enough (so it just dumps the available
>>>>>>>>>> metadata functions from the running kernel at load time), it may be
>>>>>>>>>> possible to make it generic enough that it will be forward-compatible
>>>>>>>>>> with new versions of the kernel that add new fields, which should
>>>>>>>>>> alleviate Florian's concern about keeping things in sync.
>>>>>>>>>
>>>>>>>>> Good point. I had to convert to a custom program to use the kfuncs :-(
>>>>>>>>> But your suggestion sounds good; maybe libxdp can accept some extra
>>>>>>>>> info about at which offset the user would like to place the metadata
>>>>>>>>> and the library can generate the required bytecode?
>>>>>>>>>
>>>>>>>>>> 3. It will make it harder to consume the metadata when building
>>>>>>>>>> SKBs. I
>>>>>>>>>> think the CPUMAP and veth use cases are also quite important, and that
>>>>>>>>>> we want metadata to be available for building SKBs in this path. Maybe
>>>>>>>>>> this can be resolved by having a convenient kfunc for this that can be
>>>>>>>>>> used for programs doing such redirects. E.g., you could just call
>>>>>>>>>> xdp_copy_metadata_for_skb() before doing the bpf_redirect, and that
>>>>>>>>>> would recursively expand into all the kfunc calls needed to extract
>>>>>>>>>> the
>>>>>>>>>> metadata supported by the SKB path?
>>>>>>>>>
>>>>>>>>> So this xdp_copy_metadata_for_skb will create a metadata layout that
>>>>>>>>
>>>>>>>> Can the xdp_copy_metadata_for_skb be written as a bpf prog itself?
>>>>>>>> Not sure where is the best point to specify this prog though.
>>>>>>>> Somehow during
>>>>>>>> bpf_xdp_redirect_map?
>>>>>>>> or this prog belongs to the target cpumap and the xdp prog
>>>>>>>> redirecting to this
>>>>>>>> cpumap has to write the meta layout in a way that the cpumap is
>>>>>>>> expecting?
>>>>>>>
>>>>>>> We're probably interested in triggering it from the places where xdp
>>>>>>> frames can eventually be converted into skbs?
>>>>>>> So for plain 'return XDP_PASS' and things like bpf_redirect/etc? (IOW,
>>>>>>> anything that's not XDP_DROP / AF_XDP redirect).
>>>>>>> We can probably make it magically work, and can generate
>>>>>>> kernel-digestible metadata whenever data == data_meta, but the
>>>>>>> question - should we?
>>>>>>> (need to make sure we won't regress any existing cases that are not
>>>>>>> relying on the metadata)
>>>>>>
>>>>>> Instead of having some kernel-digestible meta data, how about calling
>>>>>> another bpf prog to initialize the skb fields from the meta area after
>>>>>> __xdp_build_skb_from_frame() in the cpumap, so
>>>>>> run_xdp_set_skb_fileds_from_metadata() may be a better name.
>>>>>>
>>>>>
>>>>> I very much like this idea of calling another bpf prog to initialize the
>>>>> SKB fields from the meta area. (As a reminder, data need to come from
>>>>> meta area, because at this point the hardware RX-desc is out-of-scope).
>>>>> I'm onboard with xdp_copy_metadata_for_skb() populating the meta area.
>>>>>
>>>>> We could invoke this BPF-prog inside __xdp_build_skb_from_frame().
>>>>>
>>>>> We might need a new BPF_PROG_TYPE_XDP2SKB as this new BPF-prog
>>>>> run_xdp_set_skb_fields_from_metadata() would need both xdp_buff + SKB as
>>>>> context inputs. Right?  (Not sure, if this is acceptable with the BPF
>>>>> maintainers new rules)
>>>>>
>>>>>> The xdp_prog@rx sets the meta data and then redirect.  If the
>>>>>> xdp_prog@rx can also specify a xdp prog to initialize the skb fields
>>>>>> from the meta area, then there is no need to have a kfunc to enforce a
>>>>>> kernel-digestible layout.  Not sure what is a good way to specify this
>>>>>> xdp_prog though...
>>>>>
>>>>> The challenge of running this (BPF_PROG_TYPE_XDP2SKB) BPF-prog inside
>>>>> __xdp_build_skb_from_frame() is that it need to know howto decode the
>>>>> meta area for every device driver or XDP-prog populating this (as veth
>>>>> and cpumap can get redirected packets from multiple device drivers).
>>>>
>>>> If we have the helper to copy the data "out of" the drivers, why do we
>>>> need a second BPF program to copy data to the SKB?
>>>>
>
> IMHO the second BPF program to populate the SKB is needed to add
> flexibility and extensibility.
>
> My end-goal here is to speedup packet parsing.
> This BPF-prog should (in time) be able to update skb->transport_header
> and skb->network_header.  As I mentioned before, HW RX-hash already tell
> us the L3 and L4 protocols and in-most-cases header-len.  Even without
> HW-hints, the XDP-prog likely have parsed the packet once. This parse
> information is lost today, and redone by netstack. What about storing
> this header parse info in meta data and re-use in this new XDP2SKB hook?
>
> The reason for suggesting this BPF-prog to be a callback, associated
> with the net_device, were that HW is going to differ on what HW hints
> that HW support.  Thus, we can avoid a generic C-function that need to
> check for all the possible hints, and instead have a BPF-prog that only
> contains the code that is relevant for this net_device.

But that's exactly what the xdp_copy_metadata_for_skb() is! It's a
dynamic "BPF program" (generated as unrolled kfunc calls) just running
in the helper and stashing the results in an intermediate struct in the
metadata area. And once it's done that, we don't need *another* dynamic
BPF program to read it back out and populate the SKB, because the
intermediate format it's been stashed into is under the control of the
kernel (we just need a flag to indicate that it's there).

>>>> I.e., the XDP program calls xdp_copy_metadata_for_skb(); this invokes
>>>> each of the kfuncs needed for the metadata used by SKBs, all of which
>>>> get unrolled. The helper takes the output of these metadata-extracting
>>>> kfuncs and stores it "somewhere". This "somewhere" could well be the
>>>> metadata area; but in any case, since it's hidden away inside a helper
>>>> (or kfunc) from the calling XDP program's PoV, the helper can just stash
>>>> all the data in a fixed format, which __xdp_build_skb_from_frame() can
>>>> then just read statically. We could even make this format match the
>>>> field layout of struct sk_buff, so all we have to do is memcpy a
>>>> contiguous chunk of memory when building the SKB.
>>>
>>> +1
>
> Sorry, I think this "hiding" layout trick is going in a wrong direction.
>
> Imagine the use-case of cpumap redirect.  The physical device XDP-prog
> calls xdp_copy_metadata_for_skb() to extract info from RX-desc, then it
> calls redirect into cpumap.  On remote CPU, the xdp_frame is picked up,
> and then I want to run another XDP-prog that want to look at these
> HW-hints, and then likely call XDP_PASS which creates the SKB, also
> using these HW-hints.  I take it, it would not be possible when using
> the xdp_copy_metadata_for_skb() helper?

You're right that it should be possible to read the values back out
again later. That is totally possible with this scheme, though; the
'xdp_to_skb_metadata' is going to be in the vmlinux BTF, so an XDP
program can just read that. We can explicitly support it by using the
BTF ID as the "magic value" as you suggest, which would be fine by me.

I still think we should be using the __randomize_layout trick, though,
precisely so that BPF consumers are forced to use BTF relocations to
read it; otherwise we risk the struct layout ossifying into UAPI because
people are just going to assume it's static...

>>> I'm currently doing exactly what you're suggesting (minus matching skb layout):
>>>
>>> struct xdp_to_skb_metadata {
>>>    u32 magic; // randomized at boot
>>>    ... skb-consumable-metadata in fixed format
>>> } __randomize_layout;
>>>
>>> bpf_xdp_copy_metadata_for_skb() does bpf_xdp_adjust_meta(ctx,
>>> -sizeof(struct xdp_to_skb_metadata)) and then calls a bunch of kfuncs
>>> to fill in the actual data.
>>>
>>> Then, at __xdp_build_skb_from_frame time, I'm having a regular kernel
>>> C code that parses that 'struct xdp_to_skb_metadata'.
>>> (To be precise, I'm trying to parse the metadata from
>>> skb_metadata_set; it's called from __xdp_build_skb_from_frame, but not
>>> 100% sure that's the right place).
>>> (I also randomize the layout and magic to make sure userspace doesn't
>>> depend on it because nothing stops this packet to be routed into xsk
>>> socket..)
>> 
>> Ah, nice trick with __randomize_layout - I agree we need to do something
>> to prevent userspace from inadvertently starting to rely on this, and
>> this seems like a great solution!
>
> Sorry, I disagree where this is going.  Why do all of a sudden want to
> prevent userspace (e.g. AF_XDP) from using this data?!?

See above: I don't think we should prevent userspace from using it (and
we're not), but we should prevent the struct layout from ossifying.

> The hole exercise started with wanting to provide AF_XDP with these
> HW-hints. The hints a standard AF_XDP user wants is likely very
> similar to what the SKB user wants. Why do the AF_XDP user need to
> open code this?
>
> The BTF-ID scheme precisely allows us to expose this layout to
> userspace, and at the same time have freedom to change this in kernel
> space, as userspace must decode the BTF-layout before reading this.
> I was hoping xdp_copy_metadata_for_skb() could simply use the BTF-ID
> scheme, with the BTF-ID of struct xdp_hints_rx_common, is to too much to
> ask for?  You can just consider the BTF-ID as the magic number, as it
> will be more-or-less random per kernel build (and module load order).

As mentioned above, I would be totally fine with just having the
xdp_to_skb_metadata be part of BTF, enabling both XDP programs and
AF_XDP consumers to re-use it.

>> Look forward to seeing what the whole thing looks like in a more
>> complete form :)
>
> I'm sort of on-board with the kfuncs and unroll-tricks, if I can see
> some driver code that handles the issues of getting HW setup state
> exposed needed to decode the RX-desc format.
>
> I sense that I myself, haven't been good enough to explain/convey the
> BTF-ID scheme.  Next week, I will code some examples that demo how
> BTF-IDs can be used from BPF-progs, even as a communication channel
> between different BPF-progs (e.g. drv XDP-prog -> cpumap XDP-prog ->
> TC-BPF).

For my part at least, it's not a lack of understanding that makes me
prefer the kfunc approach. Rather, it's the complexity of having to
resolve the multiple BTF IDs, and the risk of ossifying the struct
layouts because people are going to do that wrong. Using kfuncs gives us
much more control of the API, especially if we combine it with struct
randomisation for the bits we do expose.

Translating what we've discussed above into the terms used in your patch
series, this would correspond to *only* having the xdp_metadata_common
struct exposed via BTF, and not bothering with all the other
driver-specific layouts. So an XDP/AF_XDP user that only wants to use
the metadata that's also used by the stack can just call
xdp_copy_metadata_for_skb(), and then read the resulting metadata area
(using BTF). And if someone wants to access metadata that's *not* used
by the stack, they'd have to call additional kfuncs to extract that.

And similarly, if someone wants only a subset of the metadata used by an
SKB, they can just *not* call xdp_copy_metadata_for_skb(), and instead
just call the individual kfuncs to extract just the fields they want.

I think this strikes a nice balance between the flexibility by the
kernel to change things, the flexibility of XDP consumers to request
only the data they want, and the ability for the same metadata to be
consumed at different points. WDYT?

-Toke