> From: Vishal Annapurve <vannapurve@xxxxxxxxxx> > Sent: Saturday, September 21, 2024 5:11 AM > > On Sun, Sep 15, 2024 at 11:08 PM Jason Gunthorpe <jgg@xxxxxxxxxx> 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@xxxxxxxxxx > > > > 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.
