> From: Jason Gunthorpe <jgg@xxxxxxxxxx> > Sent: Saturday, May 29, 2021 1:36 AM > > On Thu, May 27, 2021 at 07:58:12AM +0000, Tian, Kevin wrote: > > > IOASID nesting can be implemented in two ways: hardware nesting and > > software nesting. With hardware support the child and parent I/O page > > tables are walked consecutively by the IOMMU to form a nested translation. > > When it's implemented in software, the ioasid driver is responsible for > > merging the two-level mappings into a single-level shadow I/O page table. > > Software nesting requires both child/parent page tables operated through > > the dma mapping protocol, so any change in either level can be captured > > by the kernel to update the corresponding shadow mapping. > > Why? A SW emulation could do this synchronization during invalidation > processing if invalidation contained an IOVA range. In this proposal we differentiate between host-managed and user- managed I/O page tables. If host-managed, the user is expected to use map/unmap cmd explicitly upon any change required on the page table. If user-managed, the user first binds its page table to the IOMMU and then use invalidation cmd to flush iotlb when necessary (e.g. typically not required when changing a PTE from non-present to present). We expect user to use map+unmap and bind+invalidate respectively instead of mixing them together. Following this policy, map+unmap must be used in both levels for software nesting, so changes in either level are captured timely to synchronize the shadow mapping. > > I think this document would be stronger to include some "Rational" > statements in key places > Sure. I tried to provide rationale as much as possible but sometimes it's lost in a complex context like this. :) Thanks Kevin
