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

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Date:   Thu, 10 Dec 2020 09:44:03 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "open list:DMA MAPPING HELPERS" <iommu@...ts.linux-foundation.org>,
        Stephen Bates <sbates@...thlin.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA
 segments in an SGL



On 2020-12-09 9:04 p.m., Dan Williams wrote:
> On Wed, Dec 9, 2020 at 6:07 PM Logan Gunthorpe <logang@...tatee.com> wrote:
>>
>>
>>
>> On 2020-12-09 6:22 p.m., Dan Williams wrote:
>>> On Mon, Nov 9, 2020 at 8:47 AM Logan Gunthorpe <logang@...tatee.com> wrote:
>>>>
>>>>
>>>>
>>>> On 2020-11-09 2:12 a.m., 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.
>>>>
>>>> Yes, I expected this would be the unacceptable part. Any alternative ideas?
>>>
>>> Why is the SG_P2PDMA_FLAG needed as compared to checking the SGL
>>> segment-pages for is_pci_p2pdma_page()?
>>
>> Because the DMA and page segments in the SGL aren't necessarily aligned...
>>
>> The IOMMU implementations can coalesce multiple pages into fewer DMA
>> address ranges, so the page pointed to by sg->page_link may not be the
>> one that corresponds to the address in sg->dma_address for a given segment.
>>
>> If that makes sense -- it's not the easiest thing to explain.
> 
> It does...
> 
> Did someone already grab, or did you already consider the 3rd
> available bit in page_link? AFAICS only SG_CHAIN and SG_END are
> reserved. However, if you have a CONFIG_64BIT dependency for
> user-directed p2pdma that would seem to allow SG_P2PDMA_FLAG to be
> (0x4) in page_link.

Hmm, I half considered that, but I had came to the conclusion that given
the mis-alignment I shouldn't be using the page side of the SGL.
However, reconsidering now, that might actually be a reasonable option.

However, the CONFIG_64BIT dependency would have to be on all P2PDMA,
because we'd need to replace pci_p2pdma_map_sg() in all cases. I'm not
sure if this would be a restriction people care about.

Thanks,

Logan