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

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Date:   Fri, 14 Apr 2017 08:37:29 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Logan Gunthorpe <logang@...tatee.com>,
        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 Thu, 2017-04-13 at 15:22 -0600, Logan Gunthorpe wrote:
> 
> 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?

I think a bunch of non-x86 architectures but I don't know which ones
outside of powerpc.

> > 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.

Cheers,
Ben.

> Thanks,
> 
> Logan