From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Martin KaFai Lau <martin.lau@linux.dev>,
Stanislav Fomichev <sdf@google.com>
Cc: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
song@kernel.org, yhs@fb.com, john.fastabend@gmail.com,
kpsingh@kernel.org, haoluo@google.com, jolsa@kernel.org,
David Ahern <dsahern@gmail.com>, Jakub Kicinski <kuba@kernel.org>,
Willem de Bruijn <willemb@google.com>,
Jesper Dangaard Brouer <brouer@redhat.com>,
Anatoly Burakov <anatoly.burakov@intel.com>,
Alexander Lobakin <alexandr.lobakin@intel.com>,
Magnus Karlsson <magnus.karlsson@gmail.com>,
Maryam Tahhan <mtahhan@redhat.com>,
xdp-hints@xdp-project.net, netdev@vger.kernel.org,
bpf@vger.kernel.org
Subject: [xdp-hints] Re: [RFC bpf-next v2 06/14] xdp: Carry over xdp metadata into skb context
Date: Fri, 11 Nov 2022 10:37:53 +0100 [thread overview]
Message-ID: <87o7td7rwu.fsf@toke.dk> (raw)
In-Reply-To: <d403ef7d-6dfd-bcaf-6088-cff5081f49e9@linux.dev>
Martin KaFai Lau <martin.lau@linux.dev> writes:
> On 11/10/22 4:10 PM, Toke Høiland-Jørgensen wrote:
>>> The problem with AF_XDP is that, IIUC, it doesn't have a data_meta
>>> pointer in the userspace.
>>>
>>> You get an rx descriptor where the address points to the 'data':
>>> | 256 bytes headroom where metadata can go | data |
>>
>> Ah, I was missing the bit where the data pointer actually points at
>> data, not the start of the buf. Oops, my bad!
>>
>>> So you have (at most) 256 bytes of headroom, some of that might be the
>>> metadata, but you really don't know where it starts. But you know it
>>> definitely ends where the data begins.
>>>
>>> So if we have the following, we can locate skb_metadata:
>>> | 256-sizeof(skb_metadata) headroom | custom metadata | skb_metadata | data |
>>> data - sizeof(skb_metadata) will get you there
>>>
>>> But if it's the other way around, the program has to know
>>> sizeof(custom metadata) to locate skb_metadata:
>>> | 256-sizeof(skb_metadata) headroom | skb_metadata | custom metadata | data |
>>>
>>> Am I missing something here?
>>
>> Hmm, so one could argue that the only way AF_XDP can consume custom
>> metadata today is if it knows out of band what the size of it is. And if
>> it knows that, it can just skip over it to go back to the skb_metadata,
>> no?
>
> +1 I replied with a similar point in another email. I also think we
> can safely assume this.
Great!
>>
>> The only problem left then is if there were multiple XDP programs called
>> in sequence (whether before a redirect, or by libxdp chaining or tail
>> calls), and the first one resized the metadata area without the last one
>> knowing about it. For this, we could add a CLOBBER_PROGRAM_META flag to
>> the skb_metadata helper which if set will ensure that the program
>> metadata length is reset to 0?
>
> How is it different from the same xdp prog calling bpf_xdp_adjust_meta() and
> bpf_xdp_metadata_export_to_skb() multiple times. The earlier stored
> skb_metadata needs to be moved during the latter bpf_xdp_adjust_meta(). The
> latter bpf_xdp_metadata_export_to_skb() will overwrite the earlier skb_metadata.
Well, it would just be a convenience flag, so instead of doing:
metalen = ctx->data - ctx->data_meta;
if (metalen)
xdp_adjust_meta(-metalen);
bpf_xdp_metadata_export_to_skb(ctx);
you could just do:
bpf_xdp_metadata_export_to_skb(ctx, CLOBBER_PROGRAM_META);
and the kernel would do the check+move for you. But, well, the couple of
extra instructions to do the check in BPF is probably fine.
(I'm talking here about a program that wants to make sure that any
custom metadata that may have been added by an earlier program is
removed before redirecting to an XSK socket; I expect we'd want to do
something like this in the default program in libxdp).
-Toke
next prev parent reply other threads:[~2022-11-11 9:38 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-04 3:25 [xdp-hints] [RFC bpf-next v2 00/14] xdp: hints via kfuncs Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 01/14] bpf: Introduce bpf_patch Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 02/14] bpf: Support inlined/unrolled kfuncs for xdp metadata Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 03/14] veth: Introduce veth_xdp_buff wrapper for xdp_buff Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 04/14] veth: Support rx timestamp metadata for xdp Stanislav Fomichev
2022-11-09 11:21 ` [xdp-hints] " Toke Høiland-Jørgensen
2022-11-09 21:34 ` Stanislav Fomichev
2022-11-10 0:25 ` John Fastabend
2022-11-10 1:02 ` Stanislav Fomichev
2022-11-10 1:35 ` John Fastabend
2022-11-10 6:44 ` Stanislav Fomichev
2022-11-10 17:39 ` John Fastabend
2022-11-10 18:52 ` Stanislav Fomichev
2022-11-11 10:41 ` Jesper Dangaard Brouer
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 05/14] selftests/bpf: Verify xdp_metadata xdp->af_xdp path Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 06/14] xdp: Carry over xdp metadata into skb context Stanislav Fomichev
2022-11-07 22:01 ` [xdp-hints] " Martin KaFai Lau
2022-11-08 21:54 ` Stanislav Fomichev
2022-11-09 3:07 ` Martin KaFai Lau
2022-11-09 4:19 ` Martin KaFai Lau
2022-11-09 11:10 ` Toke Høiland-Jørgensen
2022-11-09 18:22 ` Martin KaFai Lau
2022-11-09 21:33 ` Stanislav Fomichev
2022-11-10 0:13 ` Martin KaFai Lau
2022-11-10 1:02 ` Stanislav Fomichev
2022-11-10 14:26 ` Toke Høiland-Jørgensen
2022-11-10 18:52 ` Stanislav Fomichev
2022-11-10 23:14 ` Toke Høiland-Jørgensen
2022-11-10 23:52 ` Stanislav Fomichev
2022-11-11 0:10 ` Toke Høiland-Jørgensen
2022-11-11 0:45 ` Martin KaFai Lau
2022-11-11 9:37 ` Toke Høiland-Jørgensen [this message]
2022-11-11 0:33 ` Martin KaFai Lau
2022-11-11 0:57 ` Stanislav Fomichev
2022-11-11 1:26 ` Martin KaFai Lau
2022-11-11 9:41 ` Toke Høiland-Jørgensen
2022-11-10 23:58 ` Martin KaFai Lau
2022-11-11 0:20 ` Stanislav Fomichev
2022-11-10 14:19 ` Toke Høiland-Jørgensen
2022-11-10 19:04 ` Martin KaFai Lau
2022-11-10 23:29 ` Toke Høiland-Jørgensen
2022-11-11 1:39 ` Martin KaFai Lau
2022-11-11 9:44 ` Toke Høiland-Jørgensen
2022-11-10 1:26 ` John Fastabend
2022-11-10 14:32 ` Toke Høiland-Jørgensen
2022-11-10 17:30 ` John Fastabend
2022-11-10 22:49 ` Toke Høiland-Jørgensen
2022-11-10 1:09 ` John Fastabend
2022-11-10 6:44 ` Stanislav Fomichev
2022-11-10 21:21 ` David Ahern
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 07/14] selftests/bpf: Verify xdp_metadata xdp->skb path Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 08/14] bpf: Helper to simplify calling kernel routines from unrolled kfuncs Stanislav Fomichev
2022-11-05 0:40 ` [xdp-hints] " Alexei Starovoitov
2022-11-05 2:18 ` Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 09/14] ice: Introduce ice_xdp_buff wrapper for xdp_buff Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 10/14] ice: Support rx timestamp metadata for xdp Stanislav Fomichev
2022-11-04 14:35 ` [xdp-hints] " Alexander Lobakin
2022-11-04 18:21 ` Stanislav Fomichev
2022-11-07 17:11 ` Alexander Lobakin
2022-11-07 19:10 ` Stanislav Fomichev
2022-12-15 11:54 ` Larysa Zaremba
2022-12-15 14:29 ` Toke Høiland-Jørgensen
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 11/14] mlx4: Introduce mlx4_xdp_buff wrapper for xdp_buff Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 12/14] mxl4: Support rx timestamp metadata for xdp Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 13/14] bnxt: Introduce bnxt_xdp_buff wrapper for xdp_buff Stanislav Fomichev
2022-11-04 3:25 ` [xdp-hints] [RFC bpf-next v2 14/14] bnxt: Support rx timestamp metadata for xdp Stanislav Fomichev
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