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

Lu Baolu
2019-07-25T07:18:44Z

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