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

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Date: Mon, 31 Jul 2023 09:23:29 +0800
From: Jason Wang <jasowang@...hat.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Christoph Hellwig <hch@...radead.org>, 
	virtualization@...ts.linux-foundation.org, 
	"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, Jul 28, 2023 at 11:03 PM 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.
> Instead of bifurcating one virtio instance into different queues why
> not create a separate virtio instance?
>

I'm not sure I get this, but do you mean a separate virtio device that
owns AF_XDP queues only? If I understand it correctly, bifurcating is
one of the key advantages of AF_XDP. What's more, current virtio
doesn't support being split at queue (pair) level. And it may still
suffer from the yes/no DMA API issue.

Thanks