Re: RFC: A KVM-specific alternative to UserfaultFD

[Peter Xu <peterx@redhat.com>]

On Mon, Nov 06, 2023 at 03:22:05PM -0800, David Matlack wrote:
> On Mon, Nov 6, 2023 at 3:03 PM Peter Xu <peterx@redhat.com> 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@redhat.com> 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.

Shouldn't the userspace normally still maintain one virtual mapping anyway
for the guest address range?  As IIUC kvm still relies a lot on HVA to work
(at least before guest memfd)? E.g., KVM_SET_USER_MEMORY_REGION, or mmu
notifiers.  If so, that 8 bytes should be there with/without userfaultfd,
IIUC.

Also, I think that's not strictly needed for any kind of file memories, as
in those case userfaultfd works with page cache.

Thanks,

-- 
Peter Xu