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From: Martin KaFai Lau <martin.lau@linux.dev>
To: Stanislav Fomichev <sdf@google.com>
Cc: "Bezdeka, Florian" <florian.bezdeka@siemens.com>,
	"kuba@kernel.org" <kuba@kernel.org>,
	"john.fastabend@gmail.com" <john.fastabend@gmail.com>,
	"alexandr.lobakin@intel.com" <alexandr.lobakin@intel.com>,
	"anatoly.burakov@intel.com" <anatoly.burakov@intel.com>,
	"song@kernel.org" <song@kernel.org>,
	"Deric, Nemanja" <nemanja.deric@siemens.com>,
	"andrii@kernel.org" <andrii@kernel.org>,
	"Kiszka, Jan" <jan.kiszka@siemens.com>,
	"magnus.karlsson@gmail.com" <magnus.karlsson@gmail.com>,
	"willemb@google.com" <willemb@google.com>,
	"ast@kernel.org" <ast@kernel.org>,
	"brouer@redhat.com" <brouer@redhat.com>,
	"yhs@fb.com" <yhs@fb.com>,
	"kpsingh@kernel.org" <kpsingh@kernel.org>,
	"daniel@iogearbox.net" <daniel@iogearbox.net>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"mtahhan@redhat.com" <mtahhan@redhat.com>,
	"xdp-hints@xdp-project.net" <xdp-hints@xdp-project.net>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"jolsa@kernel.org" <jolsa@kernel.org>,
	"haoluo@google.com" <haoluo@google.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>
Subject: [xdp-hints] Re: [RFC bpf-next 0/5] xdp: hints via kfuncs
Date: Tue, 1 Nov 2022 10:05:13 -0700	[thread overview]
Message-ID: <0c00ba33-f37b-dfe6-7980-45920ffa273b@linux.dev> (raw)
In-Reply-To: <CAKH8qBsGB1G60cu91Au816gsB2zF8T0P-yDwxbTEOxX0TN3WgA@mail.gmail.com>

On 10/31/22 6:59 PM, Stanislav Fomichev wrote:
> On Mon, Oct 31, 2022 at 3:57 PM Martin KaFai Lau <martin.lau@linux.dev> 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.

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 kernel will be able to understand when converting back to skb?
>>> IIUC, the xdp program will look something like the following:
>>>
>>> if (xdp packet is to be consumed by af_xdp) {
>>>     // do a bunch of bpf_xdp_metadata_<metadata> calls and assemble your
>>> own metadata layout
>>>     return bpf_redirect_map(xsk, ...);
>>> } else {
>>>     // if the packet is to be consumed by the kernel
>>>     xdp_copy_metadata_for_skb(ctx);
>>>     return bpf_redirect(...);
>>> }
>>>
>>> Sounds like a great suggestion! xdp_copy_metadata_for_skb can maybe
>>> put some magic number in the first byte(s) of the metadata so the
>>> kernel can check whether xdp_copy_metadata_for_skb has been called
>>> previously (or maybe xdp_frame can carry this extra signal, idk).


  parent reply	other threads:[~2022-11-01 17:05 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-27 20:00 [xdp-hints] " Stanislav Fomichev
2022-10-27 20:00 ` [xdp-hints] [RFC bpf-next 1/5] bpf: Support inlined/unrolled kfuncs for xdp metadata Stanislav Fomichev
2022-10-27 20:00 ` [xdp-hints] [RFC bpf-next 2/5] veth: Support rx timestamp metadata for xdp Stanislav Fomichev
2022-10-28  8:40   ` [xdp-hints] " Jesper Dangaard Brouer
2022-10-28 18:46     ` Stanislav Fomichev
2022-10-27 20:00 ` [xdp-hints] [RFC bpf-next 3/5] libbpf: Pass prog_ifindex via bpf_object_open_opts Stanislav Fomichev
2022-10-27 20:05   ` [xdp-hints] " Andrii Nakryiko
2022-10-27 20:10     ` Stanislav Fomichev
2022-10-27 20:00 ` [xdp-hints] [RFC bpf-next 4/5] selftests/bpf: Convert xskxceiver to use custom program Stanislav Fomichev
2022-10-27 20:00 ` [xdp-hints] [RFC bpf-next 5/5] selftests/bpf: Test rx_timestamp metadata in xskxceiver Stanislav Fomichev
2022-10-28  6:22   ` [xdp-hints] " Martin KaFai Lau
2022-10-28 10:37     ` Jesper Dangaard Brouer
2022-10-28 18:46       ` Stanislav Fomichev
2022-10-31 14:20         ` Alexander Lobakin
2022-10-31 14:29           ` Alexander Lobakin
2022-10-31 17:00           ` Stanislav Fomichev
2022-11-01 13:18             ` Jesper Dangaard Brouer
2022-11-01 20:12               ` Stanislav Fomichev
2022-11-01 22:23               ` Toke Høiland-Jørgensen
2022-10-28 15:58 ` [xdp-hints] Re: [RFC bpf-next 0/5] xdp: hints via kfuncs John Fastabend
2022-10-28 18:04   ` Jakub Kicinski
2022-10-28 18:46     ` Stanislav Fomichev
2022-10-28 23:16       ` John Fastabend
2022-10-29  1:14         ` Jakub Kicinski
2022-10-31 14:10           ` Bezdeka, Florian
2022-10-31 15:28             ` Toke Høiland-Jørgensen
2022-10-31 17:00               ` Stanislav Fomichev
2022-10-31 22:57                 ` Martin KaFai Lau
2022-11-01  1:59                   ` Stanislav Fomichev
2022-11-01 12:52                     ` Toke Høiland-Jørgensen
2022-11-01 13:43                       ` David Ahern
2022-11-01 14:20                         ` Toke Høiland-Jørgensen
2022-11-01 17:05                     ` Martin KaFai Lau [this message]
2022-11-01 20:12                       ` Stanislav Fomichev
2022-11-02 14:06                       ` Jesper Dangaard Brouer
2022-11-02 22:01                         ` Toke Høiland-Jørgensen
2022-11-02 23:10                           ` Stanislav Fomichev
2022-11-03  0:09                             ` Toke Høiland-Jørgensen
2022-11-03 12:01                               ` Jesper Dangaard Brouer
2022-11-03 12:48                                 ` Toke Høiland-Jørgensen
2022-11-03 15:25                                   ` Jesper Dangaard Brouer
2022-10-31 19:36               ` Yonghong Song
2022-10-31 22:09                 ` Stanislav Fomichev
2022-10-31 22:38                   ` Yonghong Song
2022-10-31 22:55                     ` Stanislav Fomichev
2022-11-01 14:23                       ` Jesper Dangaard Brouer
2022-11-01 17:31                   ` Martin KaFai Lau
2022-11-01 20:12                     ` Stanislav Fomichev
2022-11-01 21:17                       ` Martin KaFai Lau
2022-10-31 17:01           ` John Fastabend

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