netdev - Re: [RFC PATCH v2 02/11] netdev: implement netlink api to bind dma-buf to netdevice

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Date: Mon, 21 Aug 2023 20:38:09 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: David Ahern <dsahern@...nel.org>, Mina Almasry <almasrymina@...gle.com>, 
	Praveen Kaligineedi <pkaligineedi@...gle.com>, netdev@...r.kernel.org, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, 
	Magnus Karlsson <magnus.karlsson@...el.com>, sdf@...gle.com, 
	Willem de Bruijn <willemb@...gle.com>, Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [RFC PATCH v2 02/11] netdev: implement netlink api to bind
 dma-buf to netdevice

On Mon, Aug 21, 2023 at 5:17 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sat, 19 Aug 2023 10:18:57 -0400 Willem de Bruijn wrote:
> > Right. Many devices only allow bringing all queues down at the same time.
> >
> > Once a descriptor is posted and the ring head is written, there is no
> > way to retract that. Since waiting for the device to catch up is not
> > acceptable, the only option is to bring down the queue, right? Which
> > will imply bringing down the entire device on many devices. Not ideal,
> > but acceptable short term, imho.
> >
> > That may be an incentive for vendors to support per-queue
> > start/stop/alloc/free. Maybe the ones that support RDMA already do?
>
> Are you talking about HW devices, or virt? I thought most HW made
> in the last 10 years should be able to take down individual queues :o

That's great. This is currently mostly encapsulated device-wide behind
ndo_close, with code looping over all rx rings, say.

Taking a look at one driver, bnxt, it indeed has a per-ring
communication exchange with the device, in hwrm_ring_free_send_msg
("/* Flush rings and disable interrupts */"), which is called before
the other normal steps: napi disable, dma unmap, posted mem free,
irq_release, napi delete and ring mem free.

This is what you meant? The issue I was unsure of was quiescing the
device immediately, i.e., that hwrm_ring_free_send_msg.

I guess this means that this could all be structured on a per-queue
basis rather than from ndo_close. Would be a significant change to
many drivers, I'd imagine.