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

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Message-ID: <20171006144309.GA30803@8bytes.org>
Date:   Fri, 6 Oct 2017 16:43:09 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     David Woodhouse <dwmw2@...radead.org>, ashok.raj@...el.com,
        leedom@...lsio.com, Harsh@...lsio.com, herbert@...dor.apana.org.au,
        iommu@...ts.linux-foundation.org, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/vt-d: Fix scatterlist offset handling

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.

Regards,

	Joerg