linux-kernel - Re: [PATCH net-next v7 7/9] vhost-net: vhost-net: replace rx

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Message-ID: <20260111234112-mutt-send-email-mst@kernel.org>
Date: Sun, 11 Jan 2026 23:42:16 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Simon Schippers <simon.schippers@...dortmund.de>,
	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 7/9] vhost-net: vhost-net: replace rx_ring
 with tun/tap ring wrappers

On Mon, Jan 12, 2026 at 10:54:15AM +0800, Jason Wang wrote:
> On Fri, Jan 9, 2026 at 5:57 PM Simon Schippers
> <simon.schippers@...dortmund.de> wrote:
> >
> > On 1/9/26 07:04, Jason Wang wrote:
> > > On Thu, Jan 8, 2026 at 3:48 PM Simon Schippers
> > > <simon.schippers@...dortmund.de> wrote:
> > >>
> > >> On 1/8/26 05:38, Jason Wang wrote:
> > >>> On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
> > >>> <simon.schippers@...dortmund.de> wrote:
> > >>>>
> > >>>> Replace the direct use of ptr_ring in the vhost-net virtqueue with
> > >>>> tun/tap ring wrapper helpers. Instead of storing an rx_ring pointer,
> > >>>> the virtqueue now stores the interface type (IF_TUN, IF_TAP, or IF_NONE)
> > >>>> and dispatches to the corresponding tun/tap helpers for ring
> > >>>> produce, consume, and unconsume operations.
> > >>>>
> > >>>> Routing ring operations through the tun/tap helpers enables netdev
> > >>>> queue wakeups, which are required for upcoming netdev queue flow
> > >>>> control support shared by tun/tap and vhost-net.
> > >>>>
> > >>>> No functional change is intended beyond switching to the wrapper
> > >>>> helpers.
> > >>>>
> > >>>> Co-developed-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> > >>>> Signed-off-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> > >>>> Co-developed by: Jon Kohler <jon@...anix.com>
> > >>>> Signed-off-by: Jon Kohler <jon@...anix.com>
> > >>>> Signed-off-by: Simon Schippers <simon.schippers@...dortmund.de>
> > >>>> ---
> > >>>>  drivers/vhost/net.c | 92 +++++++++++++++++++++++++++++----------------
> > >>>>  1 file changed, 60 insertions(+), 32 deletions(-)
> > >>>>
> > >>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > >>>> index 7f886d3dba7d..215556f7cd40 100644
> > >>>> --- a/drivers/vhost/net.c
> > >>>> +++ b/drivers/vhost/net.c
> > >>>> @@ -90,6 +90,12 @@ enum {
> > >>>>         VHOST_NET_VQ_MAX = 2,
> > >>>>  };
> > >>>>
> > >>>> +enum if_type {
> > >>>> +       IF_NONE = 0,
> > >>>> +       IF_TUN = 1,
> > >>>> +       IF_TAP = 2,
> > >>>> +};
> > >>>
> > >>> This looks not elegant, can we simply export objects we want to use to
> > >>> vhost like get_tap_socket()?
> > >>
> > >> No, we cannot do that. We would need access to both the ptr_ring and the
> > >> net_device. However, the net_device is protected by an RCU lock.
> > >>
> > >> That is why {tun,tap}_ring_consume_batched() are used:
> > >> they take the appropriate locks and handle waking the queue.
> > >
> > > How about introducing a callback in the ptr_ring itself, so vhost_net
> > > only need to know about the ptr_ring?
> >
> > That would be great, but I'm not sure whether this should be the
> > responsibility of the ptr_ring.
> >
> > If the ptr_ring were to keep track of the netdev queue, it could handle
> > all the management itself - stopping the queue when full and waking it
> > again once space becomes available.
> >
> > What would be your idea for implementing this?
> 
> During ptr_ring_init() register a callback, the callback will be
> trigger during ptr_ring_consume() or ptr_ring_consume_batched() when
> ptr_ring find there's a space for ptr_ring_produce().
> 
> Thanks

Not sure the perceived elegance is worth the indirect call overhead.
ptr_ring is trying hard to be low overhead.
What this does is not really complex to justify that.
We just need decent documentation.

> >
> > >
> > > Thanks
> > >
> > >>
> > >>>
> > >>> Thanks
> > >>>
> > >>
> > >
> >