From: "Raj, Ashok" <ashok.raj@intel.com>
Subject: Re: [PATCH] iommu/vt-d: Fix scatterlist offset handling
Date: Fri, 6 Oct 2017 05:54:29 -0700
Message-ID: <20171006125428.GA258394@otc-nc-03>
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Cc: Robin Murphy <robin.murphy@arm.com>,
        David Woodhouse <dwmw2@infradead.org>, leedom@chelsio.com,
        Harsh@chelsio.com, herbert@gondor.apana.org.au,
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To: Joerg Roedel <joro@8bytes.org>
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On Fri, Oct 06, 2017 at 04:43:09PM +0200, Joerg Roedel wrote:
> 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.

That seems like a good start. I have reviewed Robin's fix and it seems
to make sense. I'll start looking at making __domain_mapping() 
to make it more manageable.

> 
> 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
>