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

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Message-ID: <0cc95df5-b9dd-6493-15fe-771d535c1020@deltatee.com>
Date:   Tue, 25 Apr 2017 11:03:27 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     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>,
        Stephen Bates <sbates@...thlin.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-nvme@...ts.infradead.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



On 25/04/17 12:30 AM, Knut Omang wrote:
> 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!

>> I also imagine it would be quite difficult to develop those models
>> given the array of hardware that needs to be supported and the deep
>> functional knowledge required to figure out appropriate restrictions.
> 
> From my naive perspective it seems it need not even be a full model to get some benefits,
> just low level functionality tests with some instances of a
> device that offers some MMIO space 'playground'.

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. We also posted [1] to qemu which was also
handy to test with using a small out of tree module. But I don't see a
lot of value in it if you completely ignore any hardware quirks.

Logan

[1] https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg04331.html