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

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Date:   Fri, 10 Feb 2023 13:31:28 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        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

From: Toke Høiland-Jørgensen <toke@...hat.com>
Date: Thu, 09 Feb 2023 21:58:07 +0100

> 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!

Let me retest both ways and will see. I saw some huge CPU loads on
reading xdpf in ice_xdp_xmit(), so that was my first thought.

> 
>> (but in bpf-next probably)
> 
> Yeah...
> 
> -Toke
>

Thanks,
Olek