linux-kernel - Re: [RFC PATCH v2 00/11] Add support to dma_map

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Message-ID: <c66d247e-5da9-4866-8e6b-ee2ec4bc03d5@deltatee.com>
Date:   Fri, 12 Mar 2021 09:18:46 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org
Cc:     Minturn Dave B <dave.b.minturn@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ira Weiny <iweiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jason Ekstrand <jason@...kstrand.net>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        Stephen Bates <sbates@...thlin.com>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Christoph Hellwig <hch@....de>,
        Xiong Jianxin <jianxin.xiong@...el.com>
Subject: Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA



On 2021-03-12 8:51 a.m., Robin Murphy wrote:
> On 2021-03-11 23:31, Logan Gunthorpe wrote:
>> Hi,
>>
>> This is a rework of the first half of my RFC for doing P2PDMA in
>> userspace
>> with O_DIRECT[1].
>>
>> The largest issue with that series was the gross way of flagging P2PDMA
>> SGL segments. This RFC proposes a different approach, (suggested by
>> Dan Williams[2]) which uses the third bit in the page_link field of the
>> SGL.
>>
>> This approach is a lot less hacky but comes at the cost of adding a
>> CONFIG_64BIT dependency to CONFIG_PCI_P2PDMA and using up the last
>> scarce bit in the page_link. For our purposes, a 64BIT restriction is
>> acceptable but it's not clear if this is ok for all usecases hoping
>> to make use of P2PDMA.
>>
>> Matthew Wilcox has already suggested (off-list) that this is the wrong
>> approach, preferring a new dma mapping operation and an SGL
>> replacement. I
>> don't disagree that something along those lines would be a better long
>> term solution, but it involves overcoming a lot of challenges to get
>> there. Creating a new mapping operation still means adding support to
>> more
>> than 25 dma_map_ops implementations (many of which are on obscure
>> architectures) or creating a redundant path to fallback with dma_map_sg()
>> for every driver that uses the new operation. This RFC is an approach
>> that doesn't require overcoming these blocks.
> 
> I don't really follow that argument - you're only adding support to two
> implementations with the awkward flag, so why would using a dedicated
> operation instead be any different? Whatever callers need to do if
> dma_pci_p2pdma_supported() says no, they could equally do if
> dma_map_p2p_sg() (or whatever) returns -ENXIO, no?

The thing is if the dma_map_sg doesn't support P2PDMA then P2PDMA
transactions cannot be done, but regular transactions can still go
through as they always did.

But replacing dma_map_sg() with dma_map_new() affects all operations,
P2PDMA or otherwise. If dma_map_new() isn't supported it can't simply
not support P2PDMA; it has to maintain a fallback path to dma_map_sg().
Given that the inputs and outputs for dma_map_new() will be completely
different data structures this will be quite a lot of similar paths
required in the driver. (ie mapping a bvec to the input struct and the
output struct to hardware requirements) If a bug crops up in the old
dma_map_sg(), developers might not notice it for some time seeing it
won't be used on the most popular architectures.

Logan