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

[Yonghong Song <yhs@meta.com>]

On 10/31/22 8:28 AM, Toke Høiland-Jørgensen wrote:
> "Bezdeka, Florian" <florian.bezdeka@siemens.com> writes:
> 
>> Hi all,
>>
>> I was closely following this discussion for some time now. Seems we
>> reached the point where it's getting interesting for me.
>>
>> On Fri, 2022-10-28 at 18:14 -0700, Jakub Kicinski wrote:
>>> On Fri, 28 Oct 2022 16:16:17 -0700 John Fastabend wrote:
>>>>>> And it's actually harder to abstract away inter HW generation
>>>>>> differences if the user space code has to handle all of it.
>>>>
>>>> I don't see how its any harder in practice though?
>>>
>>> You need to find out what HW/FW/config you're running, right?
>>> And all you have is a pointer to a blob of unknown type.
>>>
>>> Take timestamps for example, some NICs support adjusting the PHC
>>> or doing SW corrections (with different versions of hw/fw/server
>>> platforms being capable of both/one/neither).
>>>
>>> Sure you can extract all this info with tracing and careful
>>> inspection via uAPI. But I don't think that's _easier_.
>>> And the vendors can't run the results thru their validation
>>> (for whatever that's worth).
>>>
>>>>> I've had the same concern:
>>>>>
>>>>> Until we have some userspace library that abstracts all these details,
>>>>> it's not really convenient to use. IIUC, with a kptr, I'd get a blob
>>>>> of data and I need to go through the code and see what particular type
>>>>> it represents for my particular device and how the data I need is
>>>>> represented there. There are also these "if this is device v1 -> use
>>>>> v1 descriptor format; if it's a v2->use this another struct; etc"
>>>>> complexities that we'll be pushing onto the users. With kfuncs, we put
>>>>> this burden on the driver developers, but I agree that the drawback
>>>>> here is that we actually have to wait for the implementations to catch
>>>>> up.
>>>>
>>>> I agree with everything there, you will get a blob of data and then
>>>> will need to know what field you want to read using BTF. But, we
>>>> already do this for BPF programs all over the place so its not a big
>>>> lift for us. All other BPF tracing/observability requires the same
>>>> logic. I think users of BPF in general perhaps XDP/tc are the only
>>>> place left to write BPF programs without thinking about BTF and
>>>> kernel data structures.
>>>>
>>>> But, with proposed kptr the complexity lives in userspace and can be
>>>> fixed, added, updated without having to bother with kernel updates, etc.
>>>>  From my point of view of supporting Cilium its a win and much preferred
>>>> to having to deal with driver owners on all cloud vendors, distributions,
>>>> and so on.
>>>>
>>>> If vendor updates firmware with new fields I get those immediately.
>>>
>>> Conversely it's a valid concern that those who *do* actually update
>>> their kernel regularly will have more things to worry about.
>>>
>>>>> Jakub mentions FW and I haven't even thought about that; so yeah, bpf
>>>>> programs might have to take a lot of other state into consideration
>>>>> when parsing the descriptors; all those details do seem like they
>>>>> belong to the driver code.
>>>>
>>>> I would prefer to avoid being stuck on requiring driver writers to
>>>> be involved. With just a kptr I can support the device and any
>>>> firwmare versions without requiring help.
>>>
>>> 1) where are you getting all those HW / FW specs :S
>>> 2) maybe *you* can but you're not exactly not an ex-driver developer :S
>>>
>>>>> Feel free to send it early with just a handful of drivers implemented;
>>>>> I'm more interested about bpf/af_xdp/user api story; if we have some
>>>>> nice sample/test case that shows how the metadata can be used, that
>>>>> might push us closer to the agreement on the best way to proceed.
>>>>
>>>> I'll try to do a intel and mlx implementation to get a cross section.
>>>> I have a good collection of nics here so should be able to show a
>>>> couple firmware versions. It could be fine I think to have the raw
>>>> kptr access and then also kfuncs for some things perhaps.
>>>>
>>>>>> I'd prefer if we left the door open for new vendors. Punting descriptor
>>>>>> parsing to user space will indeed result in what you just said - major
>>>>>> vendors are supported and that's it.
>>>>
>>>> I'm not sure about why it would make it harder for new vendors? I think
>>>> the opposite,
>>>
>>> TBH I'm only replying to the email because of the above part :)
>>> I thought this would be self evident, but I guess our perspectives
>>> are different.
>>>
>>> Perhaps you look at it from the perspective of SW running on someone
>>> else's cloud, an being able to move to another cloud, without having
>>> to worry if feature X is available in xdp or just skb.
>>>
>>> I look at it from the perspective of maintaining a cloud, with people
>>> writing random XDP applications. If I swap a NIC from an incumbent to a
>>> (superior) startup, and cloud users are messing with raw descriptor -
>>> I'd need to go find every XDP program out there and make sure it
>>> understands the new descriptors.
>>
>> Here is another perspective:
>>
>> As AF_XDP application developer I don't wan't to deal with the
>> underlying hardware in detail. I like to request a feature from the OS
>> (in this case rx/tx timestamping). If the feature is available I will
>> simply use it, if not I might have to work around it - maybe by falling
>> back to SW timestamping.
>>
>> All parts of my application (BPF program included) should not be
>> optimized/adjusted for all the different HW variants out there.
> 
> Yes, absolutely agreed. Abstracting away those kinds of hardware
> differences is the whole *point* of having an OS/driver model. I.e.,
> it's what the kernel is there for! If people want to bypass that and get
> direct access to the hardware, they can already do that by using DPDK.
> 
> So in other words, 100% agreed that we should not expect the BPF
> developers to deal with hardware details as would be required with a
> kptr-based interface.
> 
> As for the kfunc-based interface, I think it shows some promise.
> Exposing a list of function names to retrieve individual metadata items
> instead of a struct layout is sorta comparable in terms of developer UI
> accessibility etc (IMO).

Looks like there are quite some use cases for hw_timestamp.
Do you think we could add it to the uapi like struct xdp_md?

The following is the current xdp_md:
struct xdp_md {
         __u32 data;
         __u32 data_end;
         __u32 data_meta;
         /* Below access go through struct xdp_rxq_info */
         __u32 ingress_ifindex; /* rxq->dev->ifindex */
         __u32 rx_queue_index;  /* rxq->queue_index  */

         __u32 egress_ifindex;  /* txq->dev->ifindex */
};

We could add  __u64 hw_timestamp to the xdp_md so user
can just do xdp_md->hw_timestamp to get the value.
xdp_md->hw_timestamp == 0 means hw_timestamp is not
available.

Inside the kernel, the ctx rewriter can generate code
to call driver specific function to retrieve the data.

The kfunc approach can be used to *less* common use cases?

> 
> There are three main drawbacks, AFAICT:
> 
> 1. It requires driver developers to write and maintain the code that
> generates the unrolled BPF bytecode to access the metadata fields, which
> is a non-trivial amount of complexity. Maybe this can be abstracted away
> with some internal helpers though (like, e.g., a
> bpf_xdp_metadata_copy_u64(dst, src, offset) helper which would spit out
> the required JMP/MOV/LDX instructions?
> 
> 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.
> 
> 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?
> 
> -Toke
>