linux-kernel - Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA segments in an SGL

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Date:   Mon, 9 Nov 2020 14:02:21 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>,
        Logan Gunthorpe <logang@...tatee.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>, linux-pci@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Ira Weiny <iweiny@...el.com>, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org,
        Stephen Bates <sbates@...thlin.com>,
        linux-block@...r.kernel.org, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA
 segments in an SGL

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.