linux-kernel - Re: [PATCH v2 01/10] PCI/P2PDMA: Support peer to peer memory

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Message-ID: <9B84B347-2CCF-4250-89ED-B29892E76597@raithlin.com>
Date:   Thu, 1 Mar 2018 23:14:46 +0000
From:   "Stephen  Bates" <sbates@...thlin.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Bjorn Helgaas <helgaas@...nel.org>
CC:     "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 v2 01/10] PCI/P2PDMA: Support peer to peer memory

> I'm pretty sure the spec disallows routing-to-self so doing a P2P 
> transaction in that sense isn't going to work unless the device 
> specifically supports it and intercepts the traffic before it gets to 
> the port.

This is correct. Unless the device intercepts the TLP before it hits the root-port then this would be considered a "route to self" violation and an error event would occur. The same holds for the downstream port on a PCI switch (unless route-to-self violations are disabled which violates the spec but which I have seen done in certain applications).
    
Stephen