linux-kernel - Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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Date:   Tue, 25 Apr 2017 21:23:30 +0000
From:   "Stephen  Bates" <sbates@...thlin.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Knut Omang <knut.omang@...cle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Dan Williams <dan.j.williams@...el.com>
CC:     Bjorn Helgaas <helgaas@...nel.org>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Christoph Hellwig <hch@....de>,
        "Sagi Grimberg" <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steve Wise <swise@...ngridcomputing.com>,
        "Max Gurtovoy" <maxg@...lanox.com>,
        Keith Busch <keith.busch@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        linux-scsi <linux-scsi@...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 <linux-nvdimm@...1.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

>> Yes, that's why I used 'significant'. One good thing is that given resources 
>> it can easily be done in parallel with other development, and will give additional
>> insight of some form.
>
>Yup, well if someone wants to start working on an emulated RDMA device
>that actually simulates proper DMA transfers that would be great!

Give that each RDMA vendor’s devices expose a different MMIO I don’t expect this to happen anytime soon.

> Yes, the nvme device in qemu has a CMB buffer which is a good choice to
> test with but we don't have code to use it for p2p transfers in the
>kernel so it is a bit awkward.

Note the CMB code is not in upstream QEMU, it’s in Keith’s fork [1]. I will see if I can push this upstream.

Stephen

[1] git://git.infradead.org/users/kbusch/qemu-nvme.git