[RFC PATCH 00/14] guest_memfd: Export guest_memfd files to iommufd

[Fred Griffoul]

From: Fred Griffoul <fgriffo@xxxxxxxxxxxx>

This series follows Ackerley's RFC v2, "guest_memfd alternative
providers" [1], and applies on top of it (kvm/next 7e60162b16f9 plus
his 50 patches). It answers one question his RFC leaves open: how the
IOMMU mappings of a device assigned to the guest follow guest_memfd.

A device cannot fault, and iommufd cannot pin guest_memfd pages: a
conversion to private, a punch hole, memory failure or a provider's
revoke must take a page away from the device too, and in PFN mode
there is no page to pin. So guest_memfd has to drive the device
mapping. This series proposes an interface for that, uses it in
iommufd as a demonstration, and adds the two PFN mode pieces it relies
on.

The interface
=============

A subsystem that maps guest_memfd pages, such as iommufd, uses three
functions and one callback, all in include/linux/guest_memfd.h:

  kvm_gmem_export_attach()      start following a guest_memfd file
  kvm_gmem_export_lookup_pfn()  which frame backs this page? A frame for
                                a shared page with memory, nothing for a
                                private page or a page without memory
  kvm_gmem_export_detach()      stop following it
  update(start, nr_pages)       the subsystem's callback: the answer for
                                this range may have changed, look it up
                                again and fix the mappings

No reference is taken on the frames. Instead, guest_memfd calls
update() on every change: conversions, punch holes, memory failure,
pages getting memory, and a PFN mode provider taking frames back or
giving them. The subsystem can rely on this:

  - the old frames stay valid until update() returns, so a device that
    never faults can switch each mapping in one step, to the new frame
    or to a scratch page; after it returns, the old frames are no
    longer used;
  - update() cannot fail (see the open questions);
  - update() may be called for pages that did not change;
  - it runs under guest_memfd's invalidate lock.

For the update to come before a frame is freed, a punch hole keeps the
frames until then: folios are held until update() returns, and in PFN
mode the range reads as holes until the update has run, before the
provider gets its frames back.

A PFN mode provider takes frames back with kvm_gmem_revoke(), from
Ackerley's RFC. When it gives frames to pages that had none, or makes a
read-only frame writable, it calls the new kvm_gmem_restore(), the
"counterpart to kvm_gmem_revoke()" that his sample leaves for users that
cannot fault (48/50). Nothing is taken away then, so restore only calls
update(); KVM and the VMM fault the pages in themselves.

Why not dma-buf: our RFC that backed guest_memfd with an imported
dma-buf [3] needed a dma-buf call for importers to build their own page
tables, and ranged invalidation, and Christian rejected both: importers
filling page tables is a no-go for dma-buf. A dma-buf also knows
nothing of shared and private pages, and folio mode guest_memfd needs
the same tracking as PFN mode. So the interface lives in guest_memfd.

iommufd, as a demonstration
===========================

IOMMU_IOAS_MAP_FILE accepts a guest_memfd. Its pages are not pinned:
each update() unmaps the range and maps what a lookup returns now, with
PAGE_SIZE IOPTEs, so a device that touches the range during an update
faults. Dirty tracking, the no-IOMMU mode and kernel access are
refused. This tests the interface and is not meant to be merged: real
support needs large IOPTEs cut and merged in place, IOPTEs replaced in
one step, and a scratch page for holes, all generic_pt work (Jason, on
our v2), and will be a separate iommufd series.

PFN mode pieces
===============

Two PFN mode changes come first, both raised in replies to Ackerley's
series:

  - get_pfn()'s order now means the naturally aligned block around the
    index, and KVM clamps it to a block it can map, aligned and of one
    attribute; today it can map a block over the wrong frames, or over
    shared and private pages. Invalidations of a PFN mode file cover
    the 1G-aligned blocks around their range, so that a fault that
    sized a block before the change retries.
  - get_pfn() gains an attributes output, with read only as its first
    bit, which the guest's, the VMM's and the IOMMU's mappings follow.

Open questions
==============

  - Private pages. The export gives shared pages only. That suits
    devices that are not trusted, and TDX, where a shared IOPTE on a
    private page is fatal. A trusted device (TDISP) needs private pages
    too: on AMD the IOMMU maps them, with IOPTE sizes that follow the
    RMP. The choice depends on the architecture and the device, so
    userspace would make it through iommufd, which would pass an attach
    flag asking lookups for private frames too, refused where the
    architecture cannot take it. Is an attach flag the right place for
    it?
  - update() cannot fail, but a consumer may need memory to apply one:
    iommufd allocates page tables to map a range again, and will need
    one to cut a large IOPTE. The demonstration only warns when that
    fails. Either the consumer reserves page tables when a range is
    mapped, or guest_memfd adds a prepare step before a conversion,
    which could fail and abort it. A provider's revoke cannot be
    aborted, so it would need the reservation either way. Which would
    you prefer?

Patches
=======

  1-2   PFN mode: clamp the order KVM maps, read-only frames.
  3-7   the export interface: folio mode, truncation, PFN mode, hole
        sizes, and updates when pages get memory.
  8-10  iommufd: track revocable domains, map guest_memfd files, and
        mock-domain IOVA queries for the test.
  11    kvm_gmem_restore().
  12-13 the sample provider: read-only pages, large blocks, and backing
        pages on demand and freeing them on a punch hole.
  14    gmem_conversion_iommufd_test.

Patch 2 also puts back the return after a failed kvm_gmem_get_pfn() in
arm64's gmem_abort(), which a kvm/next merge dropped; the fix alone
will be sent separately.

Testing
=======

On x86_64 in QEMU, with KASAN and lockdep:

  - gmem_conversion_iommufd_test maps the guest_memfd of a
    KVM_X86_SW_PROTECTED_VM into an iommufd mock domain, backed by
    folios, a tmpfs resource or the sample provider in PFN mode, and
    checks that the guest, the VMM and the device all see each change:
    conversions, allocations, punch holes, memory failure, and the
    provider's revokes and restores;
  - Ackerley's tests pass as on his RFC alone;
  - the iommufd selftests pass, except 12 dmabuf and
    access_domain_destory tests that also fail on his RFC alone here;
  - each patch builds with KVM and iommufd built in, with KVM as a
    module, with both as modules, and with SEV.

arm64 is neither built nor run.

History
=======

Our v2 [2] added an interface in mm for drivers that lend memory, with
guest_memfd and iommufd as two consumers of a provider file. Ackerley's
RFC now gives guest_memfd PFN mode providers itself and takes v2's
sample provider, so this series drops the mm interface: iommufd maps
the guest_memfd, not the provider file, and follows conversions.

[1] https://lore.kernel.org/all/20261022-gmem-alternative-providers-v2-0-5dd1dc6c660a@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20261014093733.90312-1-griffoul@xxxxxxxxx/
[3] https://lore.kernel.org/all/20261005095329.49764-1-griffoul@xxxxxxxxx/

prerequisite-message-id: <20261022-gmem-alternative-providers-v2-0-5dd1dc6c660a@xxxxxxxxxx>

Fred Griffoul (14):
  KVM: guest_memfd: Clamp the order of PFN mode blocks to what KVM can
    map
  KVM: guest_memfd: Support read-only frames from PFN mode providers
  KVM: guest_memfd: Allow other subsystems to map guest_memfd files
  KVM: guest_memfd: Keep truncated folios until the exports are updated
  KVM: guest_memfd: Export PFN mode files
  KVM: guest_memfd: Report the size of holes in export lookups
  KVM: guest_memfd: Update exports when pages are allocated
  iommufd: Track the domains of revocable pages in struct iopt_pages
  iommufd: Map guest_memfd files
  iommufd/selftest: Add mock-domain IOVA queries
  KVM: guest_memfd: Add kvm_gmem_restore() for PFN mode providers
  samples/kvm: Add read-only pages and large blocks to the sample
    provider
  samples/kvm: Back absent pages on demand in the sample provider
  KVM: selftests: Test guest_memfd mapped by iommufd

 arch/arm64/kvm/mmu.c                          |  10 +-
 arch/arm64/kvm/nested.c                       |   9 +-
 arch/x86/kvm/Makefile                         |   4 +-
 arch/x86/kvm/mmu/mmu.c                        |  15 +-
 arch/x86/kvm/svm/sev.c                        |   4 +-
 drivers/iommu/iommufd/io_pagetable.c          |  38 +-
 drivers/iommu/iommufd/io_pagetable.h          |  61 +-
 drivers/iommu/iommufd/iommufd_test.h          |  18 +
 drivers/iommu/iommufd/pages.c                 | 366 ++++++-
 drivers/iommu/iommufd/selftest.c              | 125 +++
 include/linux/guest_memfd.h                   |  93 +-
 include/linux/kvm_host.h                      |   6 +-
 include/uapi/linux/iommufd.h                  |  11 +-
 samples/kvm/mem_provider_sample.c             | 118 ++-
 samples/kvm/mem_provider_sample.h             |  18 +-
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../kvm/x86/gmem_conversion_iommufd_test.c    | 956 ++++++++++++++++++
 virt/kvm/guest_memfd.c                        | 601 ++++++++++-
 18 files changed, 2329 insertions(+), 125 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/x86/gmem_conversion_iommufd_test.c


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