netdev - Re: [PATCH net-next 2/2] tun: AF

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Date: Thu, 25 Jan 2024 11:44:55 +0800
From: Jason Wang <jasowang@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Yunjian Wang <wangyunjian@...wei.com>, mst@...hat.com, kuba@...nel.org, 
	davem@...emloft.net, magnus.karlsson@...el.com, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux.dev, xudingke@...wei.com
Subject: Re: [PATCH net-next 2/2] tun: AF_XDP Rx zero-copy support

On Thu, Jan 25, 2024 at 3:05 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Yunjian Wang wrote:
> > Now the zero-copy feature of AF_XDP socket is supported by some
> > drivers, which can reduce CPU utilization on the xdp program.
> > This patch set allows tun to support AF_XDP Rx zero-copy feature.
> >
> > This patch tries to address this by:
> > - Use peek_len to consume a xsk->desc and get xsk->desc length.
> > - When the tun support AF_XDP Rx zero-copy, the vq's array maybe empty.
> > So add a check for empty vq's array in vhost_net_buf_produce().
> > - add XDP_SETUP_XSK_POOL and ndo_xsk_wakeup callback support
> > - add tun_put_user_desc function to copy the Rx data to VM
> >
> > Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
>
> I don't fully understand the higher level design of this feature yet.
>
> But some initial comments at the code level.
>
> > ---
> >  drivers/net/tun.c   | 165 +++++++++++++++++++++++++++++++++++++++++++-
> >  drivers/vhost/net.c |  18 +++--
> >  2 files changed, 176 insertions(+), 7 deletions(-)
> >

[...]

> >  struct tun_page {
> > @@ -208,6 +21
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index f2ed7167c848..a1f143ad2341 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
>
> For virtio maintainer: is it okay to have tun and vhost/net changes in
> the same patch, or is it better to split them?

It's better to split, but as you comment below, if it must be done in
one patch we need to explain why.

>
> > @@ -169,9 +169,10 @@ static int vhost_net_buf_is_empty(struct vhost_net_buf *rxq)
> >
> >  static void *vhost_net_buf_consume(struct vhost_net_buf *rxq)
> >  {
> > -     void *ret = vhost_net_buf_get_ptr(rxq);
> > -     ++rxq->head;
> > -     return ret;
> > +     if (rxq->tail == rxq->head)
> > +             return NULL;
> > +
> > +     return rxq->queue[rxq->head++];
>
> Why this change?

Thanks