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

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Message-ID: <1690770875.591743-2-xuanzhuo@linux.alibaba.com>
Date: Mon, 31 Jul 2023 10:34:35 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jakub Kicinski <kuba@...nel.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>,
 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 Fri, 28 Jul 2023 08:03:05 -0700, Jakub Kicinski <kuba@...nel.org> wrote:
> On Fri, 28 Jul 2023 14:02:33 +0800 Xuan Zhuo wrote:
> > Hi guys, this topic is stuck again. How should I proceed with this work?
> >
> > Let me briefly summarize:
> > 1. The problem with adding virtio_dma_{map, sync} api is that, for AF_XDP and
> > the driver layer, we need to support these APIs. The current conclusion of
> > AF_XDP is no.
> >
> > 2. Set dma_set_mask_and_coherent, then we can use DMA API uniformly inside
> > driver. This idea seems to be inconsistent with the framework design of DMA. The
> > conclusion is no.
> >
> > 3. We noticed that if the virtio device supports VIRTIO_F_ACCESS_PLATFORM, it
> > uses DMA API. And this type of device is the future direction, so we only
> > support DMA premapped for this type of virtio device. The problem with this
> > solution is that virtqueue_dma_dev() only returns dev in some cases, because
> > VIRTIO_F_ACCESS_PLATFORM is supported in such cases. Otherwise NULL is returned.
> > This option is currently NO.
> >
> > So I'm wondering what should I do, from a DMA point of view, is there any
> > solution in case of using DMA API?
>
> I'd step back and ask you why do you want to use AF_XDP with virtio.

Or do you mean virtio vs virtio-net?
All I did with virtio was to get the virtio-net to support AF_XDP.

> Instead of bifurcating one virtio instance into different queues

That is not the key of our problem.

Even though we have a device that only works with AF_XDP,
it still has this DMA issues.

I think the current way(v11, v12) is a good solution, the only problem is that
if the device is old, we can not do dma with DMA APIs. Then we will not suppot
AF_XDP. I don't think it matters. But Christoph was a little worried.

Thanks.

> why
> not create a separate virtio instance?