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From: Logan Gunthorpe <logang@deltatee.com>
To: Keith Busch <keith.busch@intel.com>
Cc: "Jens Axboe" <axboe@kernel.dk>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	linux-nvdimm@lists.01.org, linux-rdma@vger.kernel.org,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Jason Gunthorpe" <jgg@mellanox.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Max Gurtovoy" <maxg@mellanox.com>,
	"Christoph Hellwig" <hch@lst.de>
Subject: Re: [PATCH v7 10/13] nvme-pci: Add support for P2P memory in requests
Date: Tue, 25 Sep 2018 11:41:44 -0600	[thread overview]
Message-ID: <3549a9bb-3604-1739-c008-4e1a95441333@deltatee.com> (raw)
In-Reply-To: <20180925171147.GB11822@localhost.localdomain>

Hey,

On 2018-09-25 11:11 a.m., Keith Busch wrote:
> Sorry if this was already discussed. Is there a reason the following
> pattern is not pushed to the generic dma_map_sg_attrs?
> 
> 	if (is_pci_p2pdma_page(sg_page(sg)))
> 		pci_p2pdma_map_sg(dev, sg, nents, dma_dir);
> 
> Beyond that, series looks good.

Yes, this has been discussed. It comes down to a few reasons:

1) Intrusiveness on other systems: ie. not needing to pay the cost for
every single dma_map_sg call

2) Consistency: we can add the check to dma_map_sg, but adding similar
functionality to dma_map_page, etc is difficult seeing it's hard for the
unmap operation to detect if a dma_addr_t was P2P memory to begin with.

3) Safety for developers trying to use P2P memory: Right now developers
must be careful with P2P pages and ensure they aren't mapped using other
means (ie dma_map_page). Having them check the drivers that are handling
the pages to ensure the appropriate map function is always used is and
that P2P pages aren't being mixed with regular pages is better than
developers relying on magic in dma_map_sg() and getting things wrong.

That being said, I think in the future everyone would like to move in
that direction but it means we will have to solve some difficult
problems with the existing infrastructure.

Logan

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  reply	other threads:[~2018-09-25 17:42 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-25 16:22 [PATCH v7 00/13] Copy Offload in NVMe Fabrics with P2P PCI Memory Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 01/13] PCI/P2PDMA: Support peer-to-peer memory Logan Gunthorpe
2018-09-25 17:25   ` Bart Van Assche
2018-09-25 18:09     ` Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 02/13] PCI/P2PDMA: Add sysfs group to display p2pmem stats Logan Gunthorpe
2018-09-25 17:29   ` Bart Van Assche
2018-09-25 18:15     ` Logan Gunthorpe
2018-09-25 18:31       ` Bart Van Assche
2018-09-25 18:51         ` Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to adjust the bus offset Logan Gunthorpe
2018-09-25 17:33   ` Bart Van Assche
2018-09-25 18:46     ` Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 04/13] PCI/P2PDMA: Introduce configfs/sysfs enable attribute helpers Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 05/13] docs-rst: Add a new directory for PCI documentation Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 06/13] PCI/P2PDMA: Add P2P DMA driver writer's documentation Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 07/13] block: Add PCI P2P flag for request queue and check support for requests Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 08/13] IB/core: Ensure we map P2P memory correctly in rdma_rw_ctx_[init|destroy]() Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 09/13] nvme-pci: Use PCI p2pmem subsystem to manage the CMB Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 10/13] nvme-pci: Add support for P2P memory in requests Logan Gunthorpe
2018-09-25 17:11   ` Keith Busch
2018-09-25 17:41     ` Logan Gunthorpe [this message]
2018-09-25 17:48       ` Keith Busch
2018-09-25 16:22 ` [PATCH v7 11/13] nvme-pci: Add a quirk for a pseudo CMB Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 12/13] nvmet: Introduce helper functions to allocate and free request SGLs Logan Gunthorpe
2018-09-25 16:22 ` [PATCH v7 13/13] nvmet: Optionally use PCI P2P memory Logan Gunthorpe

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