From: "Toke Høiland-Jørgensen" <toke@redhat•com>
To: Jakub Kicinski <kuba@kernel•org>
Cc: John Fastabend <john.fastabend@gmail•com>,
Alexei Starovoitov <alexei.starovoitov@gmail•com>,
Lorenzo Bianconi <lorenzo@kernel•org>, bpf <bpf@vger•kernel.org>,
Network Development <netdev@vger•kernel.org>,
Lorenzo Bianconi <lorenzo.bianconi@redhat•com>,
"David S. Miller" <davem@davemloft•net>,
Alexei Starovoitov <ast@kernel•org>,
Daniel Borkmann <daniel@iogearbox•net>,
shayagr@amazon•com, David Ahern <dsahern@kernel•org>,
Jesper Dangaard Brouer <brouer@redhat•com>,
Eelco Chaudron <echaudro@redhat•com>,
Jason Wang <jasowang@redhat•com>,
Alexander Duyck <alexander.duyck@gmail•com>,
Saeed Mahameed <saeed@kernel•org>,
"Fijalkowski, Maciej" <maciej.fijalkowski@intel•com>,
"Karlsson, Magnus" <magnus.karlsson@intel•com>,
tirthendu.sarkar@intel•com
Subject: Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support
Date: Mon, 20 Sep 2021 23:01:48 +0200 [thread overview]
Message-ID: <87lf3r3qrn.fsf@toke.dk> (raw)
In-Reply-To: <20210920110216.4c54c9a3@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Jakub Kicinski <kuba@kernel•org> writes:
> On Sat, 18 Sep 2021 13:53:35 +0200 Toke Høiland-Jørgensen wrote:
>> I'm OK with a bpf_header_pointer()-type helper - I quite like the
>> in-kernel version of this for SKBs, so replicating it as a BPF helper
>> would be great. But I'm a little worried about taking a performance hit.
>>
>> I.e., if you do:
>>
>> ptr = bpf_header_pointer(pkt, offset, len, stack_ptr)
>> *ptr = xxx;
>>
>> then, if the helper ended up copying the data into the stack pointer,
>> you didn't actually change anything in the packet, so you need to do a
>> writeback.
>>
>> Jakub suggested up-thread that this should be done with some kind of
>> flush() helper. But you don't know whether the header_pointer()-helper
>> copied the data, so you always need to call the flush() helper, which
>> will incur overhead. If the verifier can in-line the helpers that will
>> lower it, but will it be enough to make it negligible?
>
> Depends on the assumptions the program otherwise makes, right?
>
> For reading I'd expect a *layout-independent* TC program would
> replace approximately:
>
> ptr = <some_ptr>;
> if (ptr + CONST >= md->ptr_end)
> if (bpf_pull_data(md, off + CONST))
> return DROP;
> ptr = <some_ptr>;
> if (ptr + CONST >= md->ptr_end)
> return DROP; /* da hell? */
> }
>
> With this (pre-inlining):
>
> ptr = bpf_header_pointer(md, offset, len, stack);
> if (!ptr)
> return DROP;
>
> Post-inlining (assuming static validation of args to prevent wraps):
>
> if (md->ptr + args->off + args->len < md->ptr_end)
> ptr = md->ptr + args->off;
> else
> ptr = __bpf_header_pointer(md, offset, len, stack);
> if (!ptr)
> return DROP;
>
> But that's based on guesswork so perhaps I'm off base.
Yeah, that's more or less what I was thinking...
> Regarding the flush() I was expecting that most progs will not modify
> the packet (or at least won't modify most headers they load) so no
> point paying the price of tracking changes auto-magically.
... but I guess my mental model assumed that packet writes would be just
as frequent as reads, in which case it would be a win if you could amend
your example like:
> In fact I don't think there is anything infra can do better for
> flushing than the prog itself:
>
> bool mod = false;
>
> ptr = bpf_header_pointer(...);
> ...
> if (some_cond(...)) {
> change_packet(...);
> mod = true;
> }
> ...
> if (mod)
to have an additional check like:
if (mod && ptr == stack)
(or something to that effect). No?
-Toke
> bpf_header_pointer_flush();
>
>
> is simple enough.. to me.
next prev parent reply other threads:[~2021-09-20 21:03 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-10 16:14 [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 01/18] net: skbuff: add size metadata to skb_shared_info for xdp Lorenzo Bianconi
2021-09-10 16:18 ` Jesper Dangaard Brouer
2021-09-10 16:14 ` [PATCH v14 bpf-next 02/18] xdp: introduce flags field in xdp_buff/xdp_frame Lorenzo Bianconi
2021-09-10 16:19 ` Jesper Dangaard Brouer
2021-09-10 16:14 ` [PATCH v14 bpf-next 03/18] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer Lorenzo Bianconi
2021-09-20 8:25 ` Shay Agroskin
2021-09-20 8:37 ` Lorenzo Bianconi
2021-09-20 8:45 ` Shay Agroskin
2021-09-20 9:00 ` Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 04/18] net: mvneta: simplify mvneta_swbm_add_rx_fragment management Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 05/18] net: xdp: add xdp_update_skb_shared_info utility routine Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 06/18] net: marvell: rely on " Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 07/18] xdp: add multi-buff support to xdp_return_{buff/frame} Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 08/18] net: mvneta: add multi buffer support to XDP_TX Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 09/18] net: mvneta: enable jumbo frames for XDP Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 10/18] bpf: add multi-buff support to the bpf_xdp_adjust_tail() API Lorenzo Bianconi
2021-09-16 16:55 ` Jakub Kicinski
2021-09-17 10:02 ` Lorenzo Bianconi
2021-09-17 13:03 ` Jakub Kicinski
2021-09-10 16:14 ` [PATCH v14 bpf-next 11/18] bpf: introduce bpf_xdp_get_buff_len helper Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 12/18] bpf: add multi-buffer support to xdp copy helpers Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 13/18] bpf: move user_size out of bpf_test_init Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 14/18] bpf: introduce multibuff support to bpf_prog_test_run_xdp() Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 15/18] bpf: test_run: add xdp_shared_info pointer in bpf_test_finish signature Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 16/18] bpf: update xdp_adjust_tail selftest to include multi-buffer Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 17/18] net: xdp: introduce bpf_xdp_adjust_data helper Lorenzo Bianconi
2021-09-10 16:14 ` [PATCH v14 bpf-next 18/18] bpf: add bpf_xdp_adjust_data selftest Lorenzo Bianconi
2021-09-16 16:55 ` [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support Jakub Kicinski
2021-09-17 14:51 ` Lorenzo Bianconi
2021-09-17 18:33 ` Jakub Kicinski
2021-09-17 18:43 ` Alexei Starovoitov
2021-09-17 19:00 ` Jakub Kicinski
2021-09-17 19:10 ` Alexei Starovoitov
2021-09-17 22:07 ` John Fastabend
2021-09-18 11:53 ` Toke Høiland-Jørgensen
2021-09-20 18:02 ` Jakub Kicinski
2021-09-20 21:01 ` Toke Høiland-Jørgensen [this message]
2021-09-20 21:25 ` Jakub Kicinski
2021-09-20 22:44 ` Toke Høiland-Jørgensen
2021-09-21 10:03 ` Eelco Chaudron
2021-09-28 11:48 ` Magnus Karlsson
2021-09-29 10:36 ` Lorenz Bauer
2021-09-29 12:25 ` Toke Høiland-Jørgensen
2021-09-29 12:32 ` Lorenz Bauer
2021-09-29 17:48 ` Jakub Kicinski
2021-09-29 17:46 ` Jakub Kicinski
2021-09-29 10:41 ` Lorenz Bauer
2021-09-29 12:10 ` Toke Høiland-Jørgensen
2021-09-29 12:38 ` Lorenz Bauer
2021-09-29 18:54 ` Alexei Starovoitov
2021-09-29 19:22 ` Jakub Kicinski
2021-09-29 20:39 ` Toke Høiland-Jørgensen
2021-10-01 9:03 ` Lorenzo Bianconi
2021-10-01 18:35 ` Jakub Kicinski
2021-10-06 9:32 ` Lorenzo Bianconi
2021-10-06 10:08 ` Eelco Chaudron
2021-10-06 12:15 ` Lorenzo Bianconi
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