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

[Christoph Hellwig]

>  /* Check if the dev needs to go through non-identity map and unmap process.*/
>  static bool iommu_need_mapping(struct device *dev)
>  {
> -	int ret;
> -
>  	if (iommu_dummy(dev))
>  		return false;
>  
> -	ret = identity_mapping(dev);
> -	if (ret) {
> -		u64 dma_mask = *dev->dma_mask;
> -
> -		if (dev->coherent_dma_mask && dev->coherent_dma_mask < dma_mask)
> -			dma_mask = dev->coherent_dma_mask;
> -
> -		if (dma_mask >= dma_get_required_mask(dev))
> -			return false;

Don't we need to keep this bit so that we still allow the IOMMU
to act if the device has a too small DMA mask to address all memory in
the system, even if if it should otherwise be identity mapped?