netdev - Re: [PATCH vhost v11 05/10] virtio_ring: introduce virtqueue_dma

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Date: Mon, 24 Jul 2023 16:05:50 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	virtualization@...ts.linux-foundation.org,
	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org,
	bpf@...r.kernel.org
Subject: Re: [PATCH vhost v11 05/10] virtio_ring: introduce
 virtqueue_dma_dev()

On Thu, Jul 20, 2023 at 03:34:01PM +0800, Xuan Zhuo wrote:
> On Wed, 19 Jul 2023 23:57:51 -0700, Christoph Hellwig <hch@...radead.org> wrote:
> > On Thu, Jul 20, 2023 at 02:45:14PM +0800, Xuan Zhuo wrote:
> > >  virtqueue_dma_dev() return the device that working with the DMA APIs.
> > >  Then that can be used like other devices. So what is the problem.
> > >
> > >  I always think the code path without the DMA APIs is the trouble for you.
> >
> > Because we now have an API where the upper level drivers sometimes
> > see the dma device and sometimes not.
> 
> No dma device is just for the old devices.
> 
> The API without DMA dev are only compatible with older devices. We can't give up
> these old devices, but we also have to embrace new features.
> 
> > This will be abused and cause
> > trouble sooner than you can say "layering".
> 
> I don't understand what the possible trouble here is.
> 
> When no dma device, the driver just does the same thing as before.
> 
> Thanks.

Instead of skipping operations, Christoph wants wrappers that
do nothing for non dma case.

-- 
MST