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

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Message-ID: <1492169849.25766.3.camel@kernel.crashing.org>
Date:   Fri, 14 Apr 2017 21:37:29 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     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>, 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 22:16 -0600, Jason Gunthorpe wrote:
> > 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.
> 
> I'd suggest just detecting if there is any translation in bus
> addresses anywhere and just hard disabling P2P on such systems.

I object to designing a subsystem that by design cannot work on whole
categories of architectures out there.

> On modern hardware with 64 bit BARs there is very little reason to
> have translation, so I think this is a legacy feature.

No it's not.

Cheers,
Ben.