Re: DMA error when sg->offset value is greater than PAGE_SIZE in Intel IOMMU

[Casey Leedom]

| From: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
| Sent: Monday, September 25, 2017 11:45 AM
|
| On Wed, 2017-09-20 at 16:01 +0800, Herbert Xu wrote:
| > Harsh Jain <Harsh@xxxxxxxxxxx> 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