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

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Date:   Thu, 13 Apr 2017 18:26:31 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     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>,
        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, Apr 13, 2017 at 03:22:06PM -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?

Any caller of pci_add_resource_offset() uses CPU addresses different from
the PCI bus addresses (unless the offset is zero, of course).  All ACPI
platforms also support this translation (see "translation_offset"), though
in most x86 systems the offset is zero.  I'm aware of one x86 system that
was tested with a non-zero offset but I don't think it was shipped that
way.

Bjorn