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Date:   Tue, 13 Mar 2018 21:56:39 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     Stephen Bates <sbates@raithlin.com>,
        Sinan Kaya <okaya@codeaurora.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
        "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Dan Williams <dan.j.williams@intel.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Alex Williamson <alex.williamson@redhat.com>
Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory
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On Tue, Mar 13, 2018 at 05:21:20PM -0600, Logan Gunthorpe wrote:
> On 13/03/18 05:08 PM, Bjorn Helgaas wrote:
> > On Tue, Mar 13, 2018 at 10:31:55PM +0000, Stephen  Bates wrote:
> > 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.
> 
> Well we've established that we don't want to allow root ports.

I assume you want to exclude Root Ports because of multi-function
devices and the "route to self" error.  I was hoping for a reference
to that so I could learn more about it.

While I was looking for it, I found sec 6.12.1.2 (PCIe r4.0), "ACS
Functions in SR-IOV Capable and Multi-Function Devices", which seems
relevant.  It talks about "peer-to-peer Requests (between Functions of
the device)".  Thay says to me that multi-function devices can DMA
between themselves.

> But we need to, at a minimum, do two pci_upstream_bridge() calls...
> 
> Remember, we need to check that a number of devices are behind the same
> switch. So we need to find a _common_ upstream port for several devices.

I agree that peers need to have a common upstream bridge.  I think
you're saying peers need to have *two* common upstream bridges.  If I
understand correctly, requiring two common bridges is a way to ensure
that peers directly below Root Ports don't try to DMA to each other.

So I guess the first order of business is to nail down whether peers
below a Root Port are prohibited from DMAing to each other.  My
assumption, based on 6.12.1.2 and the fact that I haven't yet found
a prohibition, is that they can.

> Excluding the multifunction device case (which I don't think is
> applicable for reasons we've discussed before), this will *never* be the
> first upstream port for a given device.

If you're looking for a common upstream bridge, you don't need to make
assumptions about whether the hierarchy is conventional PCI or PCIe or
how many levels are in the hierarchy.

You already have upstream_bridges_match(), which takes two pci_devs.
I think it should walk up the PCI hierarchy from the first device,
checking whether the bridge at each level is also a parent of the
second device.

Bjorn