[RFC PATCH 0/6] KVM: guest_memfd: back guest_memfd with an imported dma-buf

[Fred Griffoul <griffoul@gmail.com>]

From: Fred Griffoul <fgriffo@amazon.co.uk>

This extends David Woodhouse's "KVM: Allow alternative providers of
guest_memfd backed by PFNMAP memory" RFC (v2) to let guest_memfd import
a dma-buf as its page source:

  https://lore.kernel.org/kvm/20260720111259.122911-1-dwmw2@infradead.org/

The memory owner is a plain dma-buf exporter, linking against neither
KVM nor iommufd. guest_memfd imports it for the guest (stage-2) plane
and iommufd imports the same buffer for the device (DMA) plane, so one
object and one invalidation protocol serve both. This is a concrete
answer to the "how do we plug into iommufd without masquerading as a
dma-buf" question from David's cover: the provider does not pretend to
be anything; the dma-buf is the shared object.

Applies on top of David's v2 series.

Patches:

 1 KVM: guest_memfd: Add a writable result to get_pfn()
   Give get_pfn() a writable out-param so a backing can map a page
   present but read-only (KVM_MEM_READONLY is rejected on guest_memfd
   slots). A guest write then exits as a memory fault. Stage-2 only;
   KVM's own host-mapped accesses are unaffected.

 2 dma-buf: Add get_phys() to describe a physical run
   New exporter op: for an offset+len, return one physically contiguous
   backed run and an attribute word (RAM/MMIO, plus READONLY). -ENOENT
   for an unbacked byte, -ENODEV when revoked. Lets a page-table-building
   importer ask where the memory is instead of taking a scatterlist.
   Converts vfio-pci, the iommufd selftest exporter and the sample;
   iommufd behaviour is unchanged.

 3 dma-buf: Add ranged mapping invalidation
   Invalidate a byte range rather than the whole buffer. The callback
   means "re-query this range" and covers loss, replacement and
   read-only flips. Importers without the callback still get a
   whole-buffer invalidation.

 4 dma-buf: Allow dynamic attach without a device
   Allow a dynamic importer with no struct device, for a consumer (KVM)
   that only needs the physical layout and builds its own tables. Such
   an importer must not map the attachment for DMA.

 5 KVM: guest_memfd: Add dma-buf backing
   Add GUEST_MEMFD_FLAG_USE_DMABUF and a dmabuf_fd to
   KVM_CREATE_GUEST_MEMFD. guest_memfd attaches as a revocable importer
   without a device, serves faults from get_phys() (largest aligned RAM
   run, read-only honoured), and zaps the range on the invalidation
   callback.

 6 samples/kvm, selftests/kvm: Exercise dma-buf backing
   Turn the sample into a dma-buf exporter (a toy memory owner: one root
   region, a child per VM, move/donate/reclaim, absent and read-only
   pages, a scratch page, per-child ioctl allowlists) and move the
   selftests to feed one dma-buf fd to both guest_memfd and iommufd.

Scope: non-confidential VMs, RAM backings only; iommufd keeps its
single-range import for now. As with David's series, this is largely
AI-assisted and is posted to get the interface shape reviewed.

Fred Griffoul (6):
  KVM: guest_memfd: Add a writable result to get_pfn()
  dma-buf: Add get_phys() to describe a physical run
  dma-buf: Add ranged mapping invalidation
  dma-buf: Allow dynamic attach without a device
  KVM: guest_memfd: Add dma-buf backing
  samples/kvm, selftests/kvm: Exercise dma-buf backing

 arch/arm64/kvm/mmu.c                          |   13 +-
 arch/arm64/kvm/nested.c                       |   10 +-
 arch/x86/kvm/mmu/mmu.c                        |   18 +-
 arch/x86/kvm/svm/sev.c                        |    4 +-
 drivers/dma-buf/dma-buf.c                     |   92 +-
 drivers/iommu/iommufd/iommufd_private.h       |    8 -
 drivers/iommu/iommufd/iommufd_test.h          |   16 +
 drivers/iommu/iommufd/pages.c                 |   74 +-
 drivers/iommu/iommufd/selftest.c              |  106 +-
 drivers/vfio/pci/vfio_pci_dmabuf.c            |   58 +-
 include/linux/dma-buf.h                       |   70 +
 include/linux/kvm_host.h                      |   12 +-
 include/linux/vfio_pci_core.h                 |    3 -
 include/trace/events/dma_buf.h                |    2 +-
 include/uapi/linux/kvm.h                      |    6 +-
 samples/kvm/gmem_provider.c                   | 1311 ++++++++++-------
 samples/kvm/gmem_provider.h                   |  144 +-
 tools/include/uapi/linux/kvm.h                |    6 +-
 tools/testing/selftests/kvm/Makefile.kvm      |    3 +-
 .../kvm/gmem_provider_nvme_dma_test.c         |   28 +-
 .../testing/selftests/kvm/include/kvm_util.h  |   25 +-
 .../testing/selftests/kvm/x86/gmem_poc_test.c |  824 +++++++++++
 .../kvm/x86/gmem_provider_hugepage_test.c     |   30 +-
 .../kvm/x86/gmem_provider_iommufd_test.c      |   42 +-
 .../kvm/x86/gmem_provider_readonly_test.c     |  172 +++
 .../kvm/x86/gmem_provider_revoke_test.c       |   34 +-
 .../selftests/kvm/x86/gmem_provider_test.c    |  190 +--
 .../kvm/x86/gmem_provider_vfio_test.c         |   45 +-
 virt/kvm/Kconfig                              |    1 +
 virt/kvm/guest_memfd.c                        |  252 +++-
 30 files changed, 2703 insertions(+), 896 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/x86/gmem_poc_test.c
 create mode 100644 tools/testing/selftests/kvm/x86/gmem_provider_readonly_test.c

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2.47.3