linux-kernel - Re: [PATCH net-next] net: stmmac: call stmmac_finalize_xdp

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Date:   Fri, 10 Mar 2023 00:08:29 +0900
From:   Leesoo Ahn <lsahn@...eel.net>
To:     Simon Horman <simon.horman@...igine.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH net-next] net: stmmac: call stmmac_finalize_xdp_rx() on a
 condition



On 23. 3. 9. 22:40, Simon Horman wrote:
> On Thu, Mar 09, 2023 at 01:26:18AM +0900, Leesoo Ahn wrote:
>> The current codebase calls the function no matter net device has XDP
>> programs or not. So the finalize function is being called everytime when RX
>> bottom-half in progress. It needs a few machine instructions for nothing
>> in the case that XDP programs are not attached at all.
>>
>> Lets it call the function on a condition that if xdp_status variable has
>> not zero value. That means XDP programs are attached to the net device
>> and it should be finalized based on the variable.
>>
>> The following instructions show that it's better than calling the function
>> unconditionally.
>>
>>    0.31 │6b8:   ldr     w0, [sp, #196]
>>         │    ┌──cbz     w0, 6cc
>>         │    │  mov     x1, x0
>>         │    │  mov     x0, x27
>>         │    │→ bl     stmmac_finalize_xdp_rx
>>         │6cc:└─→ldr    x1, [sp, #176]
>>
>> with 'if (xdp_status)' statement, jump to '6cc' label if xdp_status has
>> zero value.
>>
>> Signed-off-by: Leesoo Ahn <lsahn@...eel.net>
> Hi Leesoo,
>
> I am curious to know if you considered going a step further and using
> a static key.
>
> Link: https://www.kernel.org/doc/html/latest/staging/static-keys.html

Thank you for the review.

The function must be called for only XDP_TX or XDP_REDIRECT cases.
So using a static key doesn't look good and the commit message is not 
clear for 'why' as well.
I think that's why you suggested for using 'static key' by the latter 
reason.

I will edit the message and post v2 soon.

Best regards,
Leesoo