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

From: Joerg Roedel
Date: Fri Oct 06 2017 - 10:43:16 EST

On Tue, Oct 03, 2017 at 07:05:17PM +0100, Robin Murphy wrote:
> Now, there are indeed plenty of drivers and subsystems which do work on
> lists of explicitly single pages - anything doing some variant of
> "addr = kmap_atomic(sg_page(sg)) + sg->offset;" is easy to spot - but I
> don't think DMA API implementations are in a position to make any kind
> of assumption; nearly all of them just shut up and handle sg->length
> bytes from sg_phys(sg) without questioning the caller, and I reckon
> that's exactly what they should be doing.

I agree with that, it is not explicitly forbidden to have an
sg->offset > PAGE_SIZE and most IOMMU drivers handle this case.

So this is a problem I'd like to see resolved in the VT-d driver too. If
nobody comes up with a correct fix soon I'll apply this one and rip out
the large-page support from __domain_mapping() to make it work.

Speaking of __domain_mapping(), this function is a big unmaintainable
mess which should be split and rewritten. A clean and maintainable
rewrite can alse re-add the large-page support.