linux-kernel - Re: [PATCH net-next v2 3/3] tun: AF

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Date: Wed, 6 Mar 2024 13:32:14 +0800
From: Jason Wang <jasowang@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Yunjian Wang <wangyunjian@...wei.com>, mst@...hat.com, 
	kuba@...nel.org, bjorn@...nel.org, magnus.karlsson@...el.com, 
	jonathan.lemon@...il.com, davem@...emloft.net, bpf@...r.kernel.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux.dev, xudingke@...wei.com, liwei395@...wei.com
Subject: Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support

On Sat, Mar 2, 2024 at 2:40 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Maciej Fijalkowski wrote:
> > On Wed, Feb 28, 2024 at 07:05:56PM +0800, Yunjian Wang wrote:
> > > This patch set allows TUN to support the AF_XDP Tx zero-copy feature,
> > > which can significantly reduce CPU utilization for XDP programs.
> >
> > Why no Rx ZC support though? What will happen if I try rxdrop xdpsock
> > against tun with this patch? You clearly allow for that.
>
> This is AF_XDP receive zerocopy, right?
>
> The naming is always confusing with tun, but even though from a tun
> PoV this happens on ndo_start_xmit, it is the AF_XDP equivalent to
> tun_put_user.
>
> So the implementation is more like other device's Rx ZC.
>
> I would have preferred that name, but I think Jason asked for this
> and given tun's weird status, there is something bo said for either.
>

>From the the view of the AF_XDP userspace program, it's the TX path,
and as you said it happens on the TUN xmit path as well. When using
with a VM, it's the RX path.

So TX seems better.

Thanks