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

Tian, Kevin
2024-09-23T05:35:59Z

> From: Vishal Annapurve <vannapurve@google.com>
> Sent: Saturday, September 21, 2024 5:11 AM
>
> On Sun, Sep 15, 2024 at 11:08 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > On Fri, Aug 23, 2024 at 11:21:26PM +1000, Alexey Kardashevskiy wrote:
> > > IOMMUFD calls get_user_pages() for every mapping which will allocate
> > > shared memory instead of using private memory managed by the KVM
> and
> > > MEMFD.
> >
> > Please check this series, it is much more how I would expect this to
> > work. Use the guest memfd directly and forget about kvm in the iommufd
> code:
> >
> > https://lore.kernel.org/r/1726319158-283074-1-git-send-email-
> steven.sistare@oracle.com
> >
> > I would imagine you'd detect the guest memfd when accepting the FD and
> > then having some different path in the pinning logic to pin and get
> > the physical ranges out.
>
> According to the discussion at KVM microconference around hugepage
> support for guest_memfd [1], it's imperative that guest private memory
> is not long term pinned. Ideal way to implement this integration would
> be to support a notifier that can be invoked by guest_memfd when
> memory ranges get truncated so that IOMMU can unmap the corresponding
> ranges. Such a notifier should also get called during memory
> conversion, it would be interesting to discuss how conversion flow
> would work in this case.
>
> [1] https://lpc.events/event/18/contributions/1764/ (checkout the
> slide 12 from attached presentation)
>

Most devices don't support I/O page fault hence can only DMA to long
term pinned buffers. The notifier might be helpful for in-kernel conversion
but as a basic requirement there needs a way for IOMMUFD to call into
guest memfd to request long term pinning for a given range. That is
how I interpreted "different path" in Jason's comment.