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

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> From: Vishal Annapurve <vannapurve@xxxxxxxxxx>
> Sent: Monday, September 23, 2024 2:34 PM
> 
> On Mon, Sep 23, 2024 at 7:36 AM Tian, Kevin <kevin.tian@xxxxxxxxx> wrote:
> >
> > > 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.
> 
> Policy that is being aimed here:
> 1) guest_memfd will pin the pages backing guest memory for all users.
> 2) kvm_gmem_get_pfn users will get a locked folio with elevated
> refcount when asking for the pfn/page from guest_memfd. Users will
> drop the refcount and release the folio lock when they are done
> using/installing (e.g. in KVM EPT/IOMMU PT entries) it. This folio
> lock is supposed to be held for short durations.
> 3) Users can assume the pfn is around until they are notified by
> guest_memfd on truncation or memory conversion.
> 
> Step 3 above is already followed by KVM EPT setup logic for CoCo VMs.
> TDX VMs especially need to have secure EPT entries always mapped (once
> faulted-in) while the guest memory ranges are private.

'faulted-in' doesn't work for device DMAs (w/o IOPF).

and above is based on the assumption that CoCo VM will always
map/pin the private memory pages until a conversion happens.

Conversion is initiated by the guest so ideally the guest is responsible 
for not leaving any in-fly DMAs to the page which is being converted.

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