linux-kernel - Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

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Message-ID: <20180313230850.GA45763@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 13 Mar 2018 18:08:50 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Stephen Bates <sbates@...thlin.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Sinan Kaya <okaya@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

On Tue, Mar 13, 2018 at 10:31:55PM +0000, Stephen  Bates wrote:
> >> It sounds like you have very tight hardware expectations for this to work
> >> at this moment. You also don't want to generalize this code for others and
> >> address the shortcomings.
> >  No, that's the way the community has pushed this work
> 
> Hi Sinan
> 
> Thanks for all the input. As Logan has pointed out the switch
> requirement is something that has evolved over time based on input
> from the community. You are more than welcome to have an opinion on
> this (and you have made that opinion clear ;-)). Over time the
> patchset may evolve from its current requirements but right now we
> are aligned with the feedback from the community.

This part of the community hasn't been convinced of the need to have
two bridges, e.g., both an Upstream Port and a Downstream Port, or two
conventional PCI bridges, above the peers.

Every PCI-to-PCI bridge is required to support routing transactions
between devices on its secondary side.  Therefore, I think it is
sufficient to verify that the potential peers share a single common
upstream bridge.  This could be a conventional PCI bridge, a Switch
Downstream Port, or a Root Port.

I've seen the response that peers directly below a Root Port could not
DMA to each other through the Root Port because of the "route to self"
issue, and I'm not disputing that.  But enforcing a requirement for
two upstream bridges introduces a weird restriction on conventional
PCI topologies, makes the code hard to read, and I don't think it's
necessary.

If it *is* necessary because Root Ports and devices below them behave
differently than in conventional PCI, I think you should include a
reference to the relevant section of the spec and check directly for a
Root Port.  I would prefer that over trying to exclude Root Ports by
looking for two upstream bridges.

Bjorn