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SubjectRe: DMA error when sg->offset value is greater than PAGE_SIZE in Intel IOMMU
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On 20/09/17 09:01, Herbert Xu wrote:
> Harsh Jain <Harsh@chelsio.com> wrote:
>>
>> While debugging DMA mapping error in chelsio crypto driver we observed that when scatter/gather list received by driver has some entry with page->offset > 4096 (PAGE_SIZE). It starts giving DMA error.  Without IOMMU it works fine.
>
> This is not a bug. The network stack can and will feed us such
> SG lists.
>
>> 2) It cannot be driver's responsibilty to update received sg entries to adjust offset and page
>> because we are not the only one who directly uses received sg list.
>
> No the driver must deal with this. Having said that, if we can
> improve our driver helper interface to make this easier then we
> should do that too. What we certainly shouldn't do is to take a
> whack-a-mole approach like this patch does.

AFAICS this is entirely on intel-iommu - from a brief look it appears
that all the IOVA calculations would handle the offset correctly, but
then __domain_mapping() blindly uses sg_page() for the physical address,
so if offset is larger than a page it would end up with the DMA mapping
covering the wrong part of the buffer.

Does the diff below help?

Robin.

----->8-----
diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index b3914fce8254..2ed43d928135 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -2253,7 +2253,7 @@ static int __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
sg_res = aligned_nrpages(sg->offset, sg->length);
sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + sg->offset;
sg->dma_length = sg->length;
- pteval = page_to_phys(sg_page(sg)) | prot;
+ pteval = (sg_phys(sg) & PAGE_MASK) | prot;
phys_pfn = pteval >> VTD_PAGE_SHIFT;
}

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