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

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On Tue, Apr 18, 2023 at 02:18:52PM +0800, Xuan Zhuo wrote:
> Sorry, rethink about this, I think we maybe misunderstand something.
> First of all, let me give you a brief introduce of virtio device and pci device.
> If I make mistake, please point out.
> First, when one virtio pci device is probed, then the virtio pci driver will be
> called. Then we got one pci_device.


> Then virtio_pci_probe will alloc one new device, and register it to virtio bus
> by register_virtio_device().
> So here we have two device: pci-device and virtio-device.


> If we call DMA API inside virtio, we use the pci-device. The virtio-device is
> not used for DMA API.


> Now we want to use the virtio-device to do direct dma. The virtio-device
> is created by virtio_pci_probe() of virtio pci driver. And register to virtio
> bus. So no firmware and not iommu and the bus is virtio bus, why we can not
> change the dma_ops of virtio-device?

Because firmware doesn't know about your virtio-device.  It is just a
made up Linux concept, and the IOMMU and firmware tables for it don't
know about it.  DMA must only ever be done on actual physical
(including "physical" devices emulated by a hypervisor) devices, not
on devices made up by Linux.
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