Re: [PATCH] iommu/vt-d: Fix scatterlist offset handling

[Raj, Ashok]

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