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    SubjectRe: [PATCH 0/3] iopmem : A block device for PCIe memory
    On Tue, Oct 18, 2016 at 08:51:15PM -0700, Dan Williams wrote:
    > [ adding Ashok and David for potential iommu comments ]
    >

    Hi Dan

    Thanks for adding Ashok and David!

    >
    > I agree with the motivation and the need for a solution, but I have
    > some questions about this implementation.
    >
    > >
    > > Consumers
    > > ---------
    > >
    > > We provide a PCIe device driver in an accompanying patch that can be
    > > used to map any PCIe BAR into a DAX capable block device. For
    > > non-persistent BARs this simply serves as an alternative to using
    > > system memory bounce buffers. For persistent BARs this can serve as an
    > > additional storage device in the system.
    >
    > Why block devices? I wonder if iopmem was initially designed back
    > when we were considering enabling DAX for raw block devices. However,
    > that support has since been ripped out / abandoned. You currently
    > need a filesystem on top of a block-device to get DAX operation.
    > Putting xfs or ext4 on top of PCI-E memory mapped range seems awkward
    > if all you want is a way to map the bar for another PCI-E device in
    > the topology.
    >
    > If you're only using the block-device as a entry-point to create
    > dax-mappings then a device-dax (drivers/dax/) character-device might
    > be a better fit.
    >

    We chose a block device because we felt it was intuitive for users to
    carve up a memory region but putting a DAX filesystem on it and creating
    files on that DAX aware FS. It seemed like a convenient way to
    partition up the region and to be easily able to get the DMA address
    for the memory backing the device.

    That said I would be very keen to get other peoples thoughts on how
    they would like to see this done. And I know some people have had some
    reservations about using DAX mounted FS to do this in the past.

    >
    > > 2. Memory Segment Spacing. This patch has the same limitations that
    > > ZONE_DEVICE does in that memory regions must be spaces at least
    > > SECTION_SIZE bytes part. On x86 this is 128MB and there are cases where
    > > BARs can be placed closer together than this. Thus ZONE_DEVICE would not
    > > be usable on neighboring BARs. For our purposes, this is not an issue as
    > > we'd only be looking at enabling a single BAR in a given PCIe device.
    > > More exotic use cases may have problems with this.
    >
    > I'm working on patches for 4.10 to allow mixing multiple
    > devm_memremap_pages() allocations within the same physical section.
    > Hopefully this won't be a problem going forward.
    >

    Thanks Dan. Your patches will help address the problem of how to
    partition a /dev/dax device but they don't help the case then BARs
    themselves are small, closely spaced and non-segment aligned. However
    I think most people using iopmem will want to use reasonbly large
    BARs so I am not sure item 2 is that big of an issue.

    > I haven't yet grokked the motivation for this, but I'll go comment on
    > that separately.

    Thanks Dan!

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