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Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>,
        Bjorn Helgaas <helgaas@kernel.org>
Cc: Logan Gunthorpe <logang@deltatee.com>, Christoph Hellwig <hch@lst.de>,
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Date: Fri, 14 Apr 2017 21:37:29 +1000
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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.