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    SubjectRe: DMA error when sg->offset value is greater than PAGE_SIZE in Intel IOMMU
    Date
    | From: David Woodhouse <dwmw2@infradead.org>
    | Sent: Monday, September 25, 2017 11:45 AM
    |
    | On Wed, 2017-09-20 at 16:01 +0800, 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.
    |
    | Hm? Under what circumstances is the offset permitted to be >
    | PAGE_SIZE?

    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 ... :-)

    Casey

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