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SubjectRe: [PATCH] iommu/vt-d: Fix scatterlist offset handling
Hi Robin

I now see your patch and it does seem to be fix the problem.

On Thu, Sep 28, 2017 at 08:43:46AM -0700, Ashok Raj wrote:
> Hi Robin
>
>
> On Thu, Sep 28, 2017 at 05:59:11PM +0100, Robin Murphy wrote:
> > I hope our email server hasn't got blacklisted again... Said patch is
> > the top of this very thread we're replying on[1] - you were definitely
> > on cc :(
>
> (sg->dma_address + sg->dma_len) ----+
> sg->dma_address ---------+ |
> iov_pfn------+ | |
> | | |
> v v v
> iova: a b c d e f
> |--------|--------|--------|--------|--------|
> <...calculated....>
> [_____mapped______]
> pfn: 0 1 2 3 4 5
> |--------|--------|--------|--------|--------|
> ^ ^ ^
> | | |
> sg->page ----+ | |
> sg->offset --------------+ |
> (sg->offset + sg->length) ----------+
>
> The picture seems right. Looking at the code i'm not sure if i understand
> it correctly.
>
> pgoff = sg->offset & ~PAGE_MASK;
>
> this gets the offset past the start of page.
>
> sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + pgoff;
>
> this would set dma_address at b+off instead of starting at c+off correct?


I assumed align_nrpages() would allocate 2 pages when
the offset is over PAGE_SIZE, but it seems economical and only
allocates 1 page to cover just the sg->length bytes.

the pteval also seems correct now, since you changed to
sg_phys() that already accounts for sg->offset, so you
subtract pgoff.

- pteval = page_to_phys(sg_page(sg)) | prot;
+ pteval = (sg_phys(sg) - pgoff) | prot;


Cheers,
Ashok

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