Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free

Lu Baolu
2019-04-29T05:18:39Z

Hi Christoph,

On 4/26/19 11:04 PM, Christoph Hellwig wrote:
> On Thu, Apr 25, 2019 at 10:07:19AM +0800, Lu Baolu wrote:
>> This is not VT-d specific. It's just how generic IOMMU works.
>>
>> Normally, IOMMU works in paging mode. So if a driver issues DMA with
>> IOVA 0xAAAA0123, IOMMU can remap it with a physical address 0xBBBB0123.
>> But we should never expect IOMMU to remap 0xAAAA0123 with physical
>> address of 0xBBBB0000. That's the reason why I said that IOMMU will not
>> work there.
>
> Well, with the iommu it doesn't happen. With swiotlb it obviosuly
> can happen, so drivers are fine with it. Why would that suddenly
> become an issue when swiotlb is called from the iommu code?
>

I would say IOMMU is DMA remapping, not DMA engine. :-)

Best regards,
Lu Baolu