netdev - RE: [EXT] Re: [PATCH v2 1/1] net: fec: add initial XDP support

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Date:   Mon, 31 Oct 2022 18:34:02 +0000
From:   Shenwei Wang <shenwei.wang@....com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     "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>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "imx@...ts.linux.dev" <imx@...ts.linux.dev>,
        kernel test robot <lkp@...el.com>
Subject: RE: [EXT] Re: [PATCH v2 1/1] net: fec: add initial XDP support



> -----Original Message-----
> From: Andrew Lunn <andrew@...n.ch>
> Sent: Monday, October 31, 2022 12:10 PM
> To: Shenwei Wang <shenwei.wang@....com>
> > +                     cbd_base = rxq->bd.base;
> > +                     if (bpf->prog)
> > +                             rxq->bd.ring_size = XDP_RX_RING_SIZE;
> > +                     else
> > +                             rxq->bd.ring_size = RX_RING_SIZE;
> > +                     size = dsize * rxq->bd.ring_size;
> > +                     cbd_base = (struct bufdesc *)(((void *)cbd_base) + size);
> > +                     rxq->bd.last = (struct bufdesc *)(((void
> > + *)cbd_base) - dsize);
> 
> This does not look safe. netif_tx_disable(dev) will stop new transmissions, but
> the hardware can be busy receiving, DMAing frames, using the descriptors, etc.
> Modifying rxq->bd.last in particular seems risky. I think you need to disable the
> receiver, wait for it to indicate it really has stopped, and only then make these
> modifications.
> 

Sounds reasonable. How about moving the codes of updating ring size to the place
right after the enet reset inside the fec_restart? This should clear those risky corner
cases.

Thanks,
Shenwei

>         Andrew