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Subject: Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA
 segments in an SGL
To:     Christoph Hellwig <hch@lst.de>,
        Logan Gunthorpe <logang@deltatee.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, linux-pci@vger.kernel.org,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Ira Weiny <iweiny@intel.com>, linux-kernel@vger.kernel.org,
        linux-nvme@lists.infradead.org,
        Stephen Bates <sbates@raithlin.com>,
        linux-block@vger.kernel.org, linux-mm@kvack.org,
        iommu@lists.linux-foundation.org,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>
References: <20201106170036.18713-1-logang@deltatee.com>
 <20201106170036.18713-5-logang@deltatee.com> <20201109091258.GB28918@lst.de>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <491c26de-bda0-3266-a67d-ee2580559a54@arm.com>
Date:   Mon, 9 Nov 2020 14:02:21 +0000
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On 2020-11-09 09:12, Christoph Hellwig wrote:
> On Fri, Nov 06, 2020 at 10:00:25AM -0700, Logan Gunthorpe wrote:
>> We make use of the top bit of the dma_length to indicate a P2PDMA
>> segment.
> 
> I don't think "we" can.  There is nothing limiting the size of a SGL
> segment.

Right, the story behind ab2cbeb0ed30 ("iommu/dma: Handle SG length 
overflow better") comes immediately to mind, for one. If all the P2P 
users can agree to be in on the game then by all means implement this in 
the P2P code, but I don't think it belongs in the generic top-level 
scatterlist API.

Robin.