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

[Lu Baolu]

Hi Christoph,

On 7/25/19 1:44 PM, Christoph Hellwig wrote:
> >   /* 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?

This checking happens only when device is using an identity mapped
domain. If the device has a small DMA mask, swiotlb will be used for
high memory access.

This is supposed to be handled in dma_direct_map_page():

        if (unlikely(!dma_direct_possible(dev, dma_addr, size)) &&
            !swiotlb_map(dev, &phys, &dma_addr, size, dir, attrs)) {
                report_addr(dev, dma_addr, size);
                return DMA_MAPPING_ERROR;
        }

Best regards,
Baolu