linux-kernel - Re: [PATCH net-next] tun: use xdp_get_frame

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Message-ID: <7C311781-F3BA-4AEB-BD17-892A88192016@nutanix.com>
Date: Thu, 8 May 2025 14:24:10 +0000
From: Jon Kohler <jon@...anix.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>
CC: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jason Wang
	<jasowang@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
        "David S. Miller"
	<davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov
	<ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend
	<john.fastabend@...il.com>,
        "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next] tun: use xdp_get_frame_len()



> On May 8, 2025, at 10:16 AM, Jesper Dangaard Brouer <hawk@...nel.org> wrote:
> 
> 
> 
> On 08/05/2025 15.29, Willem de Bruijn wrote:
>> Jon Kohler wrote:
>>> 
>>> 
>>>> On May 7, 2025, at 4:56 PM, Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
>>>> 
>>>> 
>>>> Jon Kohler wrote:
>>>>> Use xdp_get_frame_len helper to ensure xdp frame size is calculated
>>>>> correctly in both single buffer and multi buffer configurations.
>>>> 
>>>> Not necessarily opposed, but multi buffer is not actually possible
>>>> in this code path, right?
>>>> 
>>>> tun_put_user_xdp only copies xdp_frame->data, for one.
>>>> 
>>>> Else this would also be fix, not net-next material.
>>> 
>>> Correct, this is a prep patch for future multi buffer support,
>>> I’m not aware of any path that can currently do that thru
>>> this code.
>>> 
> 
> This is a good example of a performance paper-cut, from my rant.
> Adding xdp_get_frame_len() where it is not needed, adds extra code,
> in-form of an if-statement and a potential touching of a colder
> cache-line in skb_shared_info area.
> 
> 
>>> The reason for pursuing multi-buffer is to allow vhost/net
>>> batching to work again for large payloads.
>> I was not aware of that context. I'd add a comment to that in the
>> commit message, and send it as part of that series.
> 
> It need to part of that series, as that batching change should bring a
> larger performance benefit that outweighs the paper-cut.

Gotcha, mission understood. Sorry for the confusion, and thank you for
taking the time to walk me through that, I appreciate it. I’ll come back to
list when the larger series is ready for eyes. 

> 
> AFAICR there is also some dual packet handling code path for XDP in
> vhost_net/tun.  I'm also willing to take the paper-cut, for cleaning
> that up.
> 
> --Jesper

When you say dual packet handling, what are you referring to specifically?