linux-kernel - Re: [PATCH net-next v7 9/9] tun/tap & vhost-net: avoid ptr

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Message-ID: <20260112061831-mutt-send-email-mst@kernel.org>
Date: Mon, 12 Jan 2026 06:19:25 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: Jason Wang <jasowang@...hat.com>, willemdebruijn.kernel@...il.com,
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, eperezma@...hat.com,
	leiyang@...hat.com, stephen@...workplumber.org, jon@...anix.com,
	tim.gebauer@...dortmund.de, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev
Subject: Re: [PATCH net-next v7 9/9] tun/tap & vhost-net: avoid ptr_ring
 tail-drop when qdisc is present

On Mon, Jan 12, 2026 at 12:17:12PM +0100, Simon Schippers wrote:
> On 1/12/26 05:33, Michael S. Tsirkin wrote:
> > On Fri, Jan 09, 2026 at 11:14:54AM +0100, Simon Schippers wrote:
> >> Am I not allowed to stop the queue and then return NETDEV_TX_BUSY?
> > 
> > We jump through a lot of hoops in virtio_net to avoid using
> > NETDEV_TX_BUSY because that bypasses all the net/ cleverness.
> > Given your patches aim to improve precisely ring full,
> > I would say stopping proactively before NETDEV_TX_BUSY
> > should be a priority.
> > 
> 
> I already proactively stop here with the approach you proposed in
> the v6.
> Or am I missing something (apart from the xdp path)?

Yes, I am just answering the general question you posed.

> 
> And yes I also dislike returning NETDEV_TX_BUSY but I do not see how
> this can be prevented with lltx enabled.

Preventing NETDEV_TX_BUSY 100% of the time is not required. It's there
to handle races.

-- 
MST