On Mon, Nov 6, 2023 at 3:03 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > On Mon, Nov 06, 2023 at 02:24:13PM -0800, Axel Rasmussen wrote: > > On Mon, Nov 6, 2023 at 12:23 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > On Mon, Nov 06, 2023 at 10:25:13AM -0800, David Matlack wrote: > > > > > > > > * Memory Overhead: UserfaultFD requires an extra 8 bytes per page of > > > > guest memory for the userspace page table entries. > > > > > > What is this one? > > > > In the way we use userfaultfd, there are two shared userspace mappings > > - one non-UFFD registered one which is used to resolve demand paging > > faults, and another UFFD-registered one which is handed to KVM et al > > for the guest to use. I think David is talking about the "second" > > mapping as overhead here, since with the KVM-based approach he's > > describing we don't need that mapping. > > I see, but then is it userspace relevant? IMHO we should discuss the > proposal based only on the design itself, rather than relying on any > details on possible userspace implementations if two mappings are not > required but optional. What I mean here is that for UserfaultFD to track accesses at PAGE_SIZE granularity, that requires 1 PTE per page, i.e. 8 bytes per page. Versus the KVM-based approach which only requires 1 bit per page for the present bitmap. This is inherent in the design of UserfaultFD because it uses PTEs to track what is present, not specific to how we use UserfaultFD.
