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

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Date:   Tue, 13 Mar 2018 21:56:39 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Stephen Bates <sbates@...thlin.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 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