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

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Message-ID: <1689838441.2670174-9-xuanzhuo@linux.alibaba.com>
Date: Thu, 20 Jul 2023 15:34:01 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Christoph Hellwig <hch@...radead.org>
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,
 "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH vhost v11 05/10] virtio_ring: introduce virtqueue_dma_dev()

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.