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Date: Fri, 31 Aug 2018 19:38:50 +0200 From: Christian König <christian.koenig@....com> To: Logan Gunthorpe <logang@...tatee.com>, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org, linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org Cc: Stephen Bates <sbates@...thlin.com>, Christoph Hellwig <hch@....de>, Keith Busch <keith.busch@...el.com>, Sagi Grimberg <sagi@...mberg.me>, Bjorn Helgaas <bhelgaas@...gle.com>, Jason Gunthorpe <jgg@...lanox.com>, Max Gurtovoy <maxg@...lanox.com>, Dan Williams <dan.j.williams@...el.com>, Jérôme Glisse <jglisse@...hat.com>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Alex Williamson <alex.williamson@...hat.com>, Jonathan Corbet <corbet@....net> Subject: Re: [PATCH v5 06/13] PCI/P2PDMA: Add P2P DMA driver writer's documentation Am 31.08.2018 um 17:51 schrieb Logan Gunthorpe: > > On 31/08/18 02:08 AM, Christian König wrote: >>> +One of the biggest issues is that PCI doesn't require forwarding >>> +transactions between hierarchy domains, and in PCIe, each Root Port >>> +defines a separate hierarchy domain. To make things worse, there is no >>> +simple way to determine if a given Root Complex supports this or not. >>> +(See PCIe r4.0, sec 1.3.1). Therefore, as of this writing, the kernel >>> +only supports doing P2P when the endpoints involved are all behind the >>> +same PCI bridge, as such devices are all in the same PCI hierarchy >>> +domain, and the spec guarantees that all transacations within the >>> +hierarchy will be routable, but it does not require routing >>> +between hierarchies. >> Can we add a kernel command line switch and a whitelist to enable P2P >> between separate hierarchies? > In future work, yes. But not for this patchset. This is definitely the > way I see things going, but we've chosen to start with what we've presented. Sounds like a plan to me. If you can separate out adding the detection I can take a look adding this with my DMA-buf P2P efforts. Christian. > > Logan
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