linux-kernel - Re: DMA error when sg->offset value is greater than PAGE

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Date:   Wed, 27 Sep 2017 18:18:02 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Casey Leedom <leedom@...lsio.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        "Harsh Jain" <Harsh@...lsio.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        Michael Werner <werner@...lsio.com>, nd@....com
Subject: Re: DMA error when sg->offset value is greater than PAGE_SIZE in
 Intel IOMMU

On Wed, 27 Sep 2017 16:31:04 +0000
Casey Leedom <leedom@...lsio.com> wrote:

> | From: Dan Williams <dan.j.williams@...el.com>
> | Sent: Tuesday, September 26, 2017 9:10 AM
> |   
> | On Tue, Sep 26, 2017 at 9:06 AM, Casey Leedom <leedom@...lsio.com>
> wrote: | > | From: Robin Murphy <robin.murphy@....com>
> | > | Sent: Tuesday, September 26, 2017 7:22 AM
> | > |...
> | > ...
> | >   Regardless, it seems that you agree that there's an issue with
> the Intel | > I/O MMU support code with regard to the legal values
> which a (struct | > scatterlist) can take on?  I still can't find any
> documentation for this | > and, personally, I'm a bit baffled by a
> Page-oriented Scatter/Gather List | > representation where [Offset,
> Offset+Length) can reside outside the Page. |
> | Consider the case where the page represents a huge page, then an
> | offset greater than PAGE_SIZE (up to HPAGE_SIZE) makes sense.
> 
>   Okay, but whatever the underlaying Page Size is, should [Offset,
> Offset+Length) completely reside within the referenced Page?  I'm just
> trying to understand the Invariance Conditions which are assumed by
> all of the code which processes Scatter/gather Lists ...

From my experience, in general terms each scatterlist segment
represents some contiguous quantity of pages, of which sg->page is the
first, while sg->length and sg->offset describe the specific bounds of
that segment's data. As such, the length may certainly (and frequently
does) exceed PAGE_SIZE; for the offset, it's unlikely that the producer
would initially construct one greater than PAGE_SIZE instead of just
pointing sg->page further forward, but it seems reasonable for it to
come about if some intermediate subsystem is processing an existing
list in-place (as seems to be the case with crypto here).

My opinion is that this may be a slightly unusual case, but I would
not consider it an illegal one. I think most DMA mapping
implementations would handle it whether intentionally or not.

Robin.