On Fri, Jun 6, 2008 at 1:21 PM, Muli Ben-Yehuda <muli@xxxxxxxxxx> wrote: .... >> It's possible to split up one flat address space and share the IOMMU >> among several users. Each user gets her own segment of bitmap and >> corresponding IO Pdir. So I don't see allocation policy as a strong >> reason to use Red/Black Tree. > > Do you mean multiple users sharing the same I/O address space (but > each user using a different segment), or multiple users, each with its > own I/O address space, but only using a specific segment of that > address space and using a single bitmap to represent free space in all > segments? Yes, I meant the former. > If the former, then you are losing some of the benefit of > the IOMMU since all users can DMA to other users areas (same I/O > address space). If the latter, having a bitmap per IO address space > seems simpler and would have the same memory consumption. Agreed. It's a trade off. ... >> I've never been able to come up with a good heuristic for >> determining the size of the IOVA space. It generally does NOT need >> to map all of Host Physical RAM. The actual requirement depends >> entirely on the workload, type and number of IO devices >> installed. The problem is we don't know any of those things until >> well after the IOMMU is already needed. > > Why not do what hash-tables implementation do, start small and resize > when we approach half-full? Historically the IOMMUs needed physically contiguous memory and resizing essentially meant quiescing all DMA, moving the IO Pdir data to the new bigger location, allocating a new bitmap and cloning the state into that as well, and then resuming DMA operations. The DMA quiesce requirement effectively meant a reboot. My understanding of Vt-d is the ranges can be added range at a time and thus can be easily resized. But it will mean more complex logic in the IOMMU bitmap handling for a domain which owns multiple bitmaps and thus a slightly higher CPU utilization cost. At least that's my guess. I'm not working on any IOMMU code lately... thanks, grant -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
