linux-kernel - Re: Enabling peer to peer device transactions for PCIe devices

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Message-ID: <20161206171213.GA870@infradead.org>
Date:   Tue, 6 Dec 2016 09:12:13 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Stephen Bates <sbates@...thlin.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Haggai Eran <haggaie@...lanox.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "Suravee.Suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "John.Bridgman@....com" <john.bridgman@....com>,
        "Alexander.Deucher@....com" <alexander.deucher@....com>,
        "Linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Max Gurtovoy <maxg@...lanox.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "serguei.sagalovitch@....com" <serguei.sagalovitch@....com>,
        "Paul.Blinzer@....com" <paul.blinzer@....com>,
        "Felix.Kuehling@....com" <felix.kuehling@....com>,
        "ben.sander@....com" <ben.sander@....com>
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On Tue, Dec 06, 2016 at 09:38:50AM -0700, Jason Gunthorpe wrote:
> > > I'm not opposed to mapping /dev/nvmeX.  However, the lookup is trivial
> > > to accomplish in sysfs through /sys/dev/char to find the sysfs path of the
> > > device-dax instance under the nvme device, or if you already have the nvme
> > > sysfs path the dax instance(s) will appear under the "dax" sub-directory.
> > 
> > Personally I think mapping the dax resource in the sysfs tree is a nice
> > way to do this and a bit more intuitive than mapping a /dev/nvmeX.
> 
> It is still not at all clear to me what userpsace is supposed to do
> with this on nvme.. How is the CMB usable from userspace?

I don't think trying to expose it to userspace makes any sense.
Exposing it to in-kernel storage targets on the other hand makes a lot
of sense.