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

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Date:   Tue, 6 Dec 2016 02:06:14 -0600
From:   "Stephen Bates" <sbates@...thlin.com>
To:     "Dan Williams" <dan.j.williams@...el.com>
Cc:     "Logan Gunthorpe" <logang@...tatee.com>,
        "Jason Gunthorpe" <jgunthorpe@...idianresearch.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

>>> I've already recommended that iopmem not be a block device and
>>> instead be a device-dax instance. I also don't think it should claim
>>> the PCI ID, rather the driver that wants to map one of its bars this
>>> way can register the memory region with the device-dax core.
>>>
>>> I'm not sure there are enough device drivers that want to do this to
>>> have it be a generic /sys/.../resource_dmableX capability. It still
>>> seems to be an exotic one-off type of configuration.
>>
>>
>> Yes, this is essentially my thinking. Except I think the userspace
>> interface should really depend on the device itself. Device dax is a
>> good  choice for many and I agree the block device approach wouldn't be
>> ideal.

I tend to agree here. The block device interface has seen quite a bit of
resistance and /dev/dax looks like a better approach for most. We can look
at doing it that way in v2.

>>
>> Specifically for NVME CMB: I think it would make a lot of sense to just
>> hand out these mappings with an mmap call on /dev/nvmeX. I expect CMB
>> buffers would be volatile and thus you wouldn't need to keep track of
>> where in the BAR the region came from. Thus, the mmap call would just be
>> an allocator from BAR memory. If device-dax were used, userspace would
>> need to lookup which device-dax instance corresponds to which nvme
>> drive.
>>
>
> 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.