Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Stephen Bates]

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!