Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free | From | Lu Baolu <> | Date | Thu, 25 Apr 2019 10:07:19 +0800 |
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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
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