linux-kernel - Re: [RFC PATCH 15/15] nvme-pci: Allow mmaping the CMB in userspace

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Date:   Mon, 9 Nov 2020 09:50:00 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Keith Busch <kbusch@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        Ira Weiny <iweiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC PATCH 15/15] nvme-pci: Allow mmaping the CMB in userspace



On 2020-11-09 8:03 a.m., Keith Busch wrote:
> On Fri, Nov 06, 2020 at 10:00:36AM -0700, Logan Gunthorpe wrote:
>> Allow userspace to obtain CMB memory by mmaping the controller's
>> char device. The mmap call allocates and returns a hunk of CMB memory,
>> (the offset is ignored) so userspace does not have control over the
>> address within the CMB.
>>
>> A VMA allocated in this way will only be usable by drivers that set
>> FOLL_PCI_P2PDMA when calling GUP. And inter-device support will be
>> checked the first time the pages are mapped for DMA.
>>
>> Currently this is only supported by O_DIRECT to an PCI NVMe device
>> or through the NVMe passthrough IOCTL.
> 
> Rather than make this be specific to nvme, could pci p2pdma create an
> mmap'able file for any resource registered with it?

It's certainly possible. However, other people have been arguing that
more of this should be specific to NVMe as some use cases do not want to
use the genalloc inside p2pdma.

Logan