Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops

[Christoph Hellwig]

On Wed, Nov 13, 2019 at 10:50:27AM +0800, Lu Baolu wrote:
> Currently, this is a block issue for using per-device dma ops in Intel
> IOMMU driver. Hence block this driver from using the generic iommu dma
> ops.

That is in fact the reason why I bring it up :)

> I'd like to align Intel IOMMU driver with other vendors. Use iommu dma
> ops for devices which have been selected to go through iommu. And use
> direct dma ops if selected to by pass.
>
> One concern of this propose is that for devices with limited address
> capability, shall we force it to use iommu or alternatively use swiotlb
> if user decides to let it by pass iommu.
>
> I understand that using swiotlb will cause some overhead due to the
> bounced buffer, but Intel IOMMU is default on hence any users who use a
> default kernel won't suffer this. We only need to document this so that
> users understand this overhead when they decide to let such devices by
> pass iommu. This is common to all vendor iommu drivers as far as I can
> see.

Indeed.  And one idea would be to lift the code in the powerpc
dma_iommu_ops that check a flag and use the direct ops to the generic
dma code and a flag in struct device.  We can then switch the intel
iommu ops (and AMD Gart) over to it.