Virtualization - Jul 2023 - [PATCH vhost v11 05/10] virtio_ring: introduce virtqueue_dma_dev()

On Fri, 28 Jul 2023 08:03:05 -0700, Jakub Kicinski <kuba at kernel.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?