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

[Vishal Annapurve <vannapurve@google.com>]

On Mon, Sep 23, 2024, 10:24 AM Tian, Kevin <kevin.tian@intel.com> wrote:
>
> > From: Vishal Annapurve <vannapurve@google.com>
> > Sent: Monday, September 23, 2024 2:34 PM
> >
> > On Mon, Sep 23, 2024 at 7:36 AM Tian, Kevin <kevin.tian@intel.com> wrote:
> > >
> > > > 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.
> >
> > 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).

faulted-in can be replaced with mapped-in for the context of IOMMU operations.

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

Host physical memory is pinned by the host software stack. If you are
talking about arch specific logic in KVM, then the expectation again
is that guest_memfd will give pinned memory to it's users.

>
> 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.
> From this angle it is fine for IOMMUFD to receive a notification from
> guest memfd when such a conversion happens.
>
> But I'm not sure whether the TDX way is architectural or just an
> implementation choice which could be changed later, or whether it
> applies to other arch.

All private memory accesses from TDX VMs go via Secure EPT. If host
removes secure EPT entries without guest intervention then linux guest
has a logic to generate a panic when it encounters EPT violation on
private memory accesses [1].

>
> If that behavior cannot be guaranteed, then we may still need a way
> for IOMMUFD to request long term pin.

[1] https://elixir.bootlin.com/linux/v6.11/source/arch/x86/coco/tdx/tdx.c#L677