> From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx> > Sent: Tuesday, October 12, 2021 4:34 PM > > On Mon, Oct 11, 2021 at 08:38:17PM -0300, Jason Gunthorpe wrote: > > On Mon, Oct 11, 2021 at 09:49:57AM +0100, Jean-Philippe Brucker wrote: > > > > > Seems like we don't need the negotiation part? The host kernel > > > communicates available IOVA ranges to userspace including holes (patch > > > 17), and userspace can check that the ranges it needs are within the IOVA > > > space boundaries. That part is necessary for DPDK as well since it needs > > > to know about holes in the IOVA space where DMA wouldn't work as > expected > > > (MSI doorbells for example). > > > > I haven't looked super closely at DPDK, but the other simple VFIO app > > I am aware of struggled to properly implement this semantic (Indeed it > > wasn't even clear to the author this was even needed). > > > > It requires interval tree logic inside the application which is not a > > trivial algorithm to implement in C. > > > > I do wonder if the "simple" interface should have an option more like > > the DMA API where userspace just asks to DMA map some user memory > and > > gets back the dma_addr_t to use. Kernel manages the allocation > > space/etc. > > Agreed, it's tempting to use IOVA = VA but the two spaces aren't > necessarily compatible. An extension that plugs into the IOVA allocator > could be useful to userspace drivers. > Make sense. We can have a flag in IOMMUFD_MAP_DMA to tell whether the user provides vaddr or expects the kernel to allocate and return. Thanks Kevin
