Re: [PATCH vhost v6 08/11] virtio_ring: introduce virtqueue_dma_dev()

From: Jason Wang <jasowang@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	virtualization@lists.linux-foundation.org,
	Guenter Roeck <linux@roeck-us.net>,
	"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH vhost v6 08/11] virtio_ring: introduce virtqueue_dma_dev()
Date: Tue, 11 Apr 2023 11:56:47 +0800	[thread overview]
Message-ID: <CACGkMEtD0BLBXv7S-TLY-Y483_Lx4UskLyPmEC2wyGa8VE_R6w@mail.gmail.com> (raw)
In-Reply-To: <ZDTUn6TUqzrF06mr@infradead.org>

On Tue, Apr 11, 2023 at 11:31 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Tue, Apr 11, 2023 at 10:09:40AM +0800, Jason Wang wrote:
> > We want to support AF_XDP for virtio-net. It means AF_XDP needs to
> > know the dma device to perform DMA mapping. So we introduce a helper
> > to expose the dma dev of the virtio device.
>
> The whole virtio architecture is based around the core code doing
> the DMA mapping.  I can't see how you can just export a helper to
> expose the dma device.  You'd have to complete rework the layering
> of the virtio code if you want to do it in the upper level drivers.
> And why would you want to do this?  The low-level code is the only
> piece that can actually know if you need to do a dma mapping.  All
> the kernel subsystems that don't do it inside the low-level drivers
> or helpers closely associtated are a giant and hard to fix map
> (see usb for the prime exhibit).
>
> So the first question is:  why do you want this for XF_ADP,

Xuan, is it possible to set up the DMA mapping inside the virtio
driver itself? I vaguely remember at least the RX buffer mapping is
done by the driver. If this is true, we can avoid exposing DMA details
to the upper layer.

> and
> the next question will be how to do that without making a complete
> mess.
>
> > This works fine as long as VIRTIO_F_ACCESS_PLATFORM is negotiated. But
> > when it is not, the virtio driver needs to use a physical address so
> > we want to expose the virtio device without dma_ops in the hope that
> > it will go for direct mapping where the physical address is used. But
> > it may not work on some specific setups (arches that assume an IOMMU
> > or have arch dma ops).
>
> The DMA device for virtio_pci is the underlying PCI device, always.
> !VIRTIO_F_ACCESS_PLATFORM means there is no dma device at all.  Because
> of all these things you can't just expose a pointer to the dma_device
> as that is just a completely wrong way of thinking about the problem.

Ok, so if there's no DMA at all we should avoid using the DMA API
completely. This means we should check dma_dev against NULL in
virtio_has_dma_quirk().

Thanks

>

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