netdev - Re: [PATCH net, stable v1 3/3] virtio_net: add checking sq is full inside xdp xmit

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Date:   Wed, 8 Mar 2023 22:51:40 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        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>,
        virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org,
        Yichun Zhang <yichun@...nresty.com>,
        Alexander Duyck <alexanderduyck@...com>, netdev@...r.kernel.org
Subject: Re: [PATCH net, stable v1 3/3] virtio_net: add checking sq is full
 inside xdp xmit

On Wed, 8 Mar 2023 07:21:07 -0500 Michael S. Tsirkin wrote:
> > >  * netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb,
> > >  *                               struct net_device *dev);
> > >  *	Called when a packet needs to be transmitted.
> > >  *	Returns NETDEV_TX_OK.  Can return NETDEV_TX_BUSY, but you should stop
> > >  *	the queue before that can happen; it's for obsolete devices and weird
> > >  *	corner cases, but the stack really does a non-trivial amount
> > >  *	of useless work if you return NETDEV_TX_BUSY.
> > >  *	Required; cannot be NULL.  
> > 
> > Thanks for the pointer. It is intersting, it seems most driver is not flollowing
> > the suggestion.  
> 
> Yes - I don't know why.

Most modern drivers don't stop the queue upfront?
We try to catch it in review, so anything remotely recent should. 
But a lot of people DTRT *and* check at the start of .xmit if 
the skb fits (return BUSY if it doesn't) - defensive programming.
But the BUSY path should never hit.