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

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Date:   Tue, 03 Oct 2017 23:16:16 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Robin Murphy <robin.murphy@....com>, joro@...tes.org
Cc:     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, 2017-10-03 at 19:05 +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.

So what's the point in sg->page in the first place? If even the
*offset* can be greater than page size, it isn't even the *first* page
(as you called it). Why aren't we just using a physical address,
instead of an arbitrary page and an offset from that?

Can we have *negative* sg->offset too? :)

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