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

[Lu Baolu]

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