linux-kernel - Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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Date:   Thu, 13 Apr 2017 15:22:06 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steve Wise <swise@...ngridcomputing.com>,
        Stephen Bates <sbates@...thlin.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     linux-pci@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...1.01.org, linux-kernel@...r.kernel.org,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory



On 12/04/17 03:55 PM, Benjamin Herrenschmidt wrote:
> Look at pcibios_resource_to_bus() and pcibios_bus_to_resource(). They
> will perform the conversion between the struct resource content (CPU
> physical address) and the actual PCI bus side address.

Ah, thanks for the tip! On my system, this translation returns the same
address so it was not necessary. And, yes, that means this would have to
find its way into the dma mapping routine somehow. This means we'll
eventually need a way to look-up the p2pmem device from the struct page.
Which means we will likely need a new flag bit in the struct page or
something. The big difficulty I see is testing. Do you know what
architectures or in what circumstances are these translations used?

> When behind the same switch you need to use PCI addresses. If one tries
> later to do P2P between host bridges (via the CPU fabric) things get
> more complex and one will have to use either CPU addresses or something
> else alltogether (probably would have to teach the arch DMA mapping
> routines to work with those struct pages you create and return the
> right thing).

Probably for starters we'd want to explicitly deny cases between host
bridges and add that later if someone wants to do the testing.

Thanks,

Logan