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SubjectRe: DMA error when sg->offset value is greater than PAGE_SIZE in Intel IOMMU
| From: David Woodhouse <>
| Sent: Monday, September 25, 2017 11:45 AM
| On Wed, 2017-09-20 at 16:01 +0800, Herbert Xu wrote:
| > Harsh Jain <> 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.
| Hm? Under what circumstances is the offset permitted to be >

As I noted earlier, this is an area of the kernel with which I'm not super
familiar. Both Herbert Xu and our local VM Expert have said that having
Scatter/Gather Lists with Offsets greater than Page Size is not a bug ...

I'm mostly trying to help out keeping focus on this because Harsh is in
India (presumable enjoying a good night's sleep while we look at this.
Hopefully we'll have a present of a bug fix for him when he wakes up ... :-)


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