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Message-ID: <a189444b-15c9-8069-901d-8cdf9af7fc3c@linux.intel.com>
Date: Thu, 25 Apr 2019 10:07:19 +0800
From: Lu Baolu <baolu.lu@...ux.intel.com>
To: Christoph Hellwig <hch@....de>
Cc: baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
Joerg Roedel <joro@...tes.org>, ashok.raj@...el.com,
jacob.jun.pan@...el.com, alan.cox@...el.com, kevin.tian@...el.com,
mika.westerberg@...ux.intel.com, pengfei.xu@...el.com,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
Marek Szyprowski <m.szyprowski@...sung.com>,
Robin Murphy <robin.murphy@....com>,
iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free
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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