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

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Message-ID: <20230728080305.5fe3737c@kernel.org>
Date: Fri, 28 Jul 2023 08:03:05 -0700
From: Jakub Kicinski <kuba@...nel.org>
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>, 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 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?