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

Lu Baolu
2019-04-25T02:13:37Z

Hi,

On 4/24/19 10:45 PM, Christoph Hellwig wrote:
> On Tue, Apr 23, 2019 at 03:32:16PM +0800, Lu Baolu wrote:
>> When we add the bounce buffer between IOVA and physical buffer, the
>> bounced buffer must starts from the same offset in a page, otherwise,
>> IOMMU can't work here.
>
> Why? Even with the odd hardware descriptors typical in Intel land that
> only allow offsets in the first page, not the following ones, having
> a zero offset where we previously had one should be fine.
>

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.

swiotlb System
IOVA bounce page Memory
.---------. .---------. .---------.
| | | | | |
| | | | | |
buffer_start .---------. .---------.buffer_start .---------.
| |----->| | | |
| | | |************>| |
| | | | swiotlb | |
IOMMU Page '---------' '---------' mapping '---------'
Boundary | | | |
| | | |
| | | |
| |----------------------------->| |
| | IOMMU mapping | |
| | | |
IOMMU Page .---------. .---------.
Boundary | | | |
| | | |
| |----------------------------->| |
| | IOMMU mapping | |
| | | |
| | | |
IOMMU Page .---------. .---------. .---------.
Boundary | | | | | |
| | | |************>| |
| |----->| | swiotlb | |
buffer_end '---------' '---------' mapping '---------'
| | | | | |
| | | | | |
'---------' '---------' '---------'

This is the whole view of iommu bounce page. I expect the buffer_start
returned by swiotlb_tbl_map_single() starts from the same page_offset as
the buffer_start in IOVA.

Best regards,
Lu Baolu