XDP hardware hints discussion mail archive
 help / color / mirror / Atom feed
From: Martin KaFai Lau <martin.lau@linux.dev>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: Stanislav Fomichev <sdf@google.com>,
	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: Thu, 10 Nov 2022 17:39:00 -0800	[thread overview]
Message-ID: <ed37045f-eb3d-8db0-4e5d-12bf7da8587e@linux.dev> (raw)
In-Reply-To: <87mt8yxuag.fsf@toke.dk>

On 11/10/22 3:29 PM, Toke Høiland-Jørgensen wrote:
>>> For the metadata consumed by the stack right now it's a bit
>>> hypothetical, yeah. However, there's a bunch of metadata commonly
>>> supported by hardware that the stack currently doesn't consume and that
>>> hopefully this feature will end up making more accessible. My hope is
>>> that the stack can also learn how to use this in the future, in which
>>> case we may run out of space. So I think of that bit mostly as
>>> future-proofing...
>>
>> ic. in this case, Can the btf_id be added to 'struct xdp_to_skb_metadata' later
>> if it is indeed needed?  The 'struct xdp_to_skb_metadata' is not in UAPI and
>> doing it with CO-RE is to give us flexibility to make this kind of changes in
>> the future.
> 
> My worry is mostly that it'll be more painful to add it later than just
> including it from the start, mostly because of AF_XDP users. But if we
> do the randomisation thing (thus forcing AF_XDP users to deal with the
> dynamic layout as well), it should be possible to add it later, and I
> can live with that option as well...

imo, considering we are trying to optimize unnecessary field initialization as 
below, it is sort of wasteful to always initialize the btf_id with the same 
value.  It is better to add it in the future when there is a need.

>>>>> We should probably also have a flag set on the xdp_frame so the stack
>>>>> knows that the metadata area contains relevant-to-skb data, to guard
>>>>> against an XDP program accidentally hitting the "magic number" (BTF_ID)
>>>>> in unrelated stuff it puts into the metadata area.
>>>>
>>>> Yeah, I think having a flag is useful.  The flag will be set at xdp_buff and
>>>> then transfer to the xdp_frame?
>>>
>>> Yeah, exactly!
>>>
>>>>>> After re-reading patch 6, have another question. The 'void
>>>>>> bpf_xdp_metadata_export_to_skb();' function signature. Should it at
>>>>>> least return ok/err? or even return a 'struct xdp_to_skb_metadata *'
>>>>>> pointer and the xdp prog can directly read (or even write) it?
>>>>>
>>>>> Hmm, I'm not sure returning a failure makes sense? Failure to read one
>>>>> or more fields just means that those fields will not be populated? We
>>>>> should probably have a flags field inside the metadata struct itself to
>>>>> indicate which fields are set or not, but I'm not sure returning an
>>>>> error value adds anything? Returning a pointer to the metadata field
>>>>> might be convenient for users (it would just be an alias to the
>>>>> data_meta pointer, but the verifier could know its size, so the program
>>>>> doesn't have to bounds check it).
>>>>
>>>> If some hints are not available, those hints should be initialized to
>>>> 0/CHECKSUM_NONE/...etc.
>>>
>>> The problem with that is that then we have to spend cycles writing
>>> eight bytes of zeroes into the checksum field :)
>>
>> With a common 'struct xdp_to_skb_metadata', I am not sure how some of these zero
>> writes can be avoided.  If the xdp prog wants to optimize, it can call
>> individual kfunc to get individual hints.
> 
> Erm, we just... don't write those fields? Something like:
> 
> void write_skb_meta(hw, ctx) {
>    struct xdp_skb_metadata meta = ctx->data_meta - sizeof(struct xdp_skb_metadata);
>    meta->valid_fields = 0;
> 
>    if (hw_has_timestamp) {
>      meta->timestamp = hw->timestamp;
>      meta->valid_fields |= FIELD_TIMESTAMP;
>    } /* otherwise meta->timestamp is just uninitialised */
> 
>    if (hw_has_rxhash) {
>      meta->rxhash = hw->rxhash;
>      meta->valid_fields |= FIELD_RXHASH;
>    } /* otherwise meta->rxhash is just uninitialised */
>    ...etc...
> }

Ah, got it.  Make sense.  My mind was stalled in the paradigm that a helper that 
needs to initialize the result.

  reply	other threads:[~2022-11-11  1:39 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
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 [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: https://lists.xdp-project.net/postorius/lists/xdp-hints.xdp-project.net/

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=ed37045f-eb3d-8db0-4e5d-12bf7da8587e@linux.dev \
    --to=martin.lau@linux.dev \
    --cc=alexandr.lobakin@intel.com \
    --cc=anatoly.burakov@intel.com \
    --cc=andrii@kernel.org \
    --cc=ast@kernel.org \
    --cc=bpf@vger.kernel.org \
    --cc=brouer@redhat.com \
    --cc=daniel@iogearbox.net \
    --cc=dsahern@gmail.com \
    --cc=haoluo@google.com \
    --cc=john.fastabend@gmail.com \
    --cc=jolsa@kernel.org \
    --cc=kpsingh@kernel.org \
    --cc=kuba@kernel.org \
    --cc=magnus.karlsson@gmail.com \
    --cc=mtahhan@redhat.com \
    --cc=netdev@vger.kernel.org \
    --cc=sdf@google.com \
    --cc=song@kernel.org \
    --cc=toke@redhat.com \
    --cc=willemb@google.com \
    --cc=xdp-hints@xdp-project.net \
    --cc=yhs@fb.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox