linux-kernel - RE: [PATCH net-next] net: fec: add XDP

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Message-ID: <AM5PR04MB313958BDA681DA11B5A9FC0B883FA@AM5PR04MB3139.eurprd04.prod.outlook.com>
Date:   Fri, 21 Jul 2023 02:29:30 +0000
From:   Wei Fang <wei.fang@....com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        Clark Wang <xiaoning.wang@....com>,
        Shenwei Wang <shenwei.wang@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: RE: [PATCH net-next] net: fec: add XDP_TX feature support

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: 2023年7月20日 23:25
> To: Wei Fang <wei.fang@....com>
> Cc: davem@...emloft.net; edumazet@...gle.com; pabeni@...hat.com;
> ast@...nel.org; daniel@...earbox.net; hawk@...nel.org;
> john.fastabend@...il.com; Clark Wang <xiaoning.wang@....com>; Shenwei
> Wang <shenwei.wang@....com>; netdev@...r.kernel.org; dl-linux-imx
> <linux-imx@....com>; linux-kernel@...r.kernel.org; bpf@...r.kernel.org
> Subject: Re: [PATCH net-next] net: fec: add XDP_TX feature support
> 
> On Thu, 20 Jul 2023 07:06:05 +0000 Wei Fang wrote:
> > > Are you taking budget into account? When NAPI is called with budget
> > > of 0 we are *not* in napi / softirq context. You can't be processing
> > > any XDP tx under such conditions (it may be a netpoll call from IRQ
> context).
> >
> > Actually, the fec driver never takes the budget into account for
> > cleaning up tx BD ring. The budget is only valid for rx.
> 
> I know, that's what I'm complaining about. XDP can only run in normal NAPI
> context, i.e. when NAPI is called with budget != 0. That works out without any
> changes on Rx, if budget is zero drivers already don't process Rx. But similar
> change must be done on Tx when adding XDP support. You can still process all
> normal skb packets on Tx when budget is 0 (in fact you should), but you
> _can't_ process any XDP Tx frame.
Sorry, I did not realize that we can not process any tx XDP packet if the "budget"
is 0. I noticed your latest clarification [1] in napi.rst, I believe it will help many
people avoid this problem like me. Thank you very much.
[1]: https://lore.kernel.org/netdev/20230720161323.2025379-1-kuba@kernel.org/T/