Re: [RFC PATCH 12/21] KVM: IOMMUFD: MEMFD: Map private pages

[Vishal Annapurve <vannapurve@google.com>]

On Tue, Sep 24, 2024 at 2:07 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> On Mon, Sep 23, 2024 at 11:52:19PM +0000, Tian, Kevin wrote:
> > > IMHO we should try to do as best we can here, and the ideal interface
> > > would be a notifier to switch the shared/private pages in some portion
> > > of the guestmemfd. With the idea that iommufd could perhaps do it
> > > atomically.
> >
> > yes atomic replacement is necessary here, as there might be in-fly
> > DMAs to pages adjacent to the one being converted in the same
> > 1G hunk. Unmap/remap could potentially break it.
>
> Yeah.. This integration is going to be much more complicated than I
> originally thought about. It will need the generic pt stuff as the
> hitless page table manipulations we are contemplating here are pretty
> complex.
>
> Jason

 To ensure that I understand your concern properly, the complexity of
handling hitless page manipulations is because guests can convert
memory at smaller granularity than the physical page size used by the
host software. Complexity remains the same irrespective of whether
kvm/guest_memfd is notifying iommu driver to unmap converted ranges or
if its userspace notifying iommu driver.