From mboxrd@z Thu Jan  1 00:00:00 1970
From: benh@kernel.crashing.org (Benjamin Herrenschmidt)
Date: Fri, 14 Apr 2017 21:37:29 +1000
Subject: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory
In-Reply-To: <20170414041656.GA30694@obsidianresearch.com>
References: <1490911959-5146-1-git-send-email-logang@deltatee.com>
 <1491974532.7236.43.camel@kernel.crashing.org>
 <5ac22496-56ec-025d-f153-140001d2a7f9@deltatee.com>
 <1492034124.7236.77.camel@kernel.crashing.org>
 <81888a1e-eb0d-cbbc-dc66-0a09c32e4ea2@deltatee.com>
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 <20170414041656.GA30694@obsidianresearch.com>
Message-ID: <1492169849.25766.3.camel@kernel.crashing.org>

On Thu, 2017-04-13@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.