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. Thanks, Jean
