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! -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
