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

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Date:   Sat, 24 Mar 2018 15:28:43 +0000
From:   "Stephen  Bates" <sbates@...thlin.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>
CC:     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

> That would be very nice but many devices do not support the internal
> route. 

But Logan in the NVMe case we are discussing movement within a single function (i.e. from a NVMe namespace to a NVMe CMB on the same function). Bjorn is discussing movement between two functions (PFs or VFs) in the same PCIe EP. In the case of multi-function endpoints I think the standard requires those devices to support internal DMAs for transfers between those functions (but does not require it within a function).

So I think the summary is:

1. There is no requirement for a single function to support internal DMAs but in the case of NVMe we do have a protocol specific way for a NVMe function to indicate it supports via the CMB BAR. Other protocols may also have such methods but I am not aware of them at this time.

2. For multi-function end-points I think it is a requirement that DMAs *between* functions are supported via an internal path but this can be over-ridden by ACS when supported in the EP.

3. For multi-function end-points there is no requirement to support internal DMA within each individual function (i.e. a la point 1 but extended to each function in a MF device). 

Based on my review of the specification I concur with Bjorn that p2pdma between functions in a MF end-point should be assured to be supported via the standard. However if the p2pdma involves only a single function in a MF device then we can only support NVMe CMBs for now. Let's review and see what the options are for supporting this in the next respin.

Stephen