linux-kernel - Re: [PATCH bpf] bpf, test_run: fix &xdp_frame misplacement for LIVE

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Date:   Thu, 09 Feb 2023 21:58:07 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf] bpf, test_run: fix &xdp_frame misplacement for
 LIVE_FRAMES

Alexander Lobakin <alexandr.lobakin@...el.com> writes:

> From: Alexander Lobakin <alexandr.lobakin@...el.com>
> Date: Thu, 9 Feb 2023 18:28:27 +0100
>
>> &xdp_buff and &xdp_frame are bound in a way that
>> 
>> xdp_buff->data_hard_start == xdp_frame
>
> [...]
>
>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>> index 2723623429ac..c3cce7a8d47d 100644
>> --- a/net/bpf/test_run.c
>> +++ b/net/bpf/test_run.c
>> @@ -97,8 +97,11 @@ static bool bpf_test_timer_continue(struct bpf_test_timer *t, int iterations,
>>  struct xdp_page_head {
>>  	struct xdp_buff orig_ctx;
>>  	struct xdp_buff ctx;
>> -	struct xdp_frame frm;
>> -	u8 data[];
>> +	union {
>> +		/* ::data_hard_start starts here */
>> +		DECLARE_FLEX_ARRAY(struct xdp_frame, frm);
>> +		DECLARE_FLEX_ARRAY(u8, data);
>> +	};
>
> BTW, xdp_frame here starts at 112 byte offset, i.e. in 16 bytes a
> cacheline boundary is hit, so xdp_frame gets sliced into halves: 16
> bytes in CL1 + 24 bytes in CL2. Maybe we'd better align this union to
> %NET_SKB_PAD / %SMP_CACHE_BYTES / ... to avoid this?

Hmm, IIRC my reasoning was that both those cache lines will be touched
by the code in xdp_test_run_batch(), so it wouldn't matter? But if
there's a performance benefit I don't mind adding an explicit alignment
annotation, certainly!

> (but in bpf-next probably)

Yeah...

-Toke