Re: [PATCH 2/3] iommu/vt-d: Apply per-device dma_ops

[Christoph Hellwig]

Hi Lu,

I really do like the switch to the per-device dma_map_ops, but:

On Thu, Aug 01, 2019 at 02:01:55PM +0800, Lu Baolu wrote:
> Current Intel IOMMU driver sets the system level dma_ops. This
> implementation has at least the following drawbacks: 1) each
> dma API will go through the IOMMU driver even the devices are
> using identity mapped domains; 2) if user requests to use an
> identity mapped domain (a.k.a. bypass iommu translation), the
> driver might fall back to dma domain blindly if the device is
> not able to address all system memory.

This is very clearly a behavioral regression.  The intel-iommu driver
has always used the iommu mapping to provide decent support for
devices that do not have the full 64-bit addressing capability, and
changing this will make a lot of existing setups go slower.

I don't think having to use swiotlb for these devices helps anyone.