On Tue, Oct 18, 2016 at 08:51:15PM -0700, Dan Williams wrote:
[ adding Ashok and David for potential iommu comments ]
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.
> 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
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