Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA

From: Logan Gunthorpe <logang@deltatee.com>
To: Robin Murphy <robin.murphy@arm.com>,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-mm@kvack.org, iommu@lists.linux-foundation.org
Cc: "Minturn Dave B" <dave.b.minturn@intel.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Ira Weiny" <iweiny@intel.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Jason Ekstrand" <jason@jlekstrand.net>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Stephen Bates" <sbates@raithlin.com>,
	"Jakowski Andrzej" <andrzej.jakowski@intel.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Xiong Jianxin" <jianxin.xiong@intel.com>
Subject: Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA
Date: Fri, 12 Mar 2021 09:18:46 -0700	[thread overview]
Message-ID: <c66d247e-5da9-4866-8e6b-ee2ec4bc03d5@deltatee.com> (raw)
In-Reply-To: <6b9be188-1ec7-527c-ae47-3f5b4e153758@arm.com>

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