For dedicated hosting environments, there are a few reasons to prefer using external PFNMAP memory for guests, rather than kernel-managed memory:
• No struct page overheads • Easily managed in 1GiB pages (or larger chunks); no fragmentation/THP • Faster kexec/KHO live update (every millisecond spent faffing around with memory management is an extra millisecond of steal time taken from guests) Today, there is only one implementation of guest_memfd: the internal shmem-based page cache in virt/kvm/guest_memfd.c. This series allows for other code to provide "a guest_memfd". This is an early path- finding proof of concept tested with AMD SEV-SNP as well as non-CoCo guests on x86 and arm64. I'm planning to build a memory-based file system which gives files for both guest_memfd as well as memory-based storage to allow things like userspace VM and orchestrator state to be passed over KHO/kexec. This series lifts guest_memfd's operations into a small ops struct and converts KVM's own gmem to use it, so it becomes one implementation among peers rather than the only game in town. It also adapts the SEV-SNP fast paths to page-less operation, plumbs iommufd to map a guest_memfd-backing dma-buf as CPU RAM (not MMIO), and adds the provider->KVM revocation primitive that makes overcommit-style reclaim possible. It's largely AI-built at the moment. My main design concerns are around how we tell that a given file is "a guest_memfd", and how we plug into IOMMUFD. Presenting a dma-buf was the less intrusive choice because that path already has support for taking pages away, but I'm less convinced that it's the cleaner choice in the long term. I'll continue to bikeshed it myself, but this is at least functional enough to show the direction and solicit further opinions... https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=gmem-provider-v2 Changes since v1 (mostly Sashiko feedback): Core (KVM / SEV): - populate: drop the transient page reference after ->post_populate() rather than before, so the callback still sees the ref it relies on - kvm_gmem_invalidate_range(): walk every address space and all memslots in the range, not just address space 0 / a single slot - SEV: the guest_memfd cache flush hit a WARN_ON_ONCE on a failed temporary mapping; that path is reachable from userspace (hole-punch), so downgrade to pr_warn_ratelimited to avoid a panic_on_warn DoS - Export file_is_kvm() (EXPORT_SYMBOL_GPL) so an external provider can confirm the fd handed to it at setup really is a KVM instance Sample provider (samples/kvm/gmem_provider.c): - Serialize per-fd state (info->kvm, bound_slot, present bitmap) under a new info->lock; v1 accessed info->kvm without locking, racing bind/unbind against release - Validate setup.kvm_fd via file_is_kvm() before using it - Fix bitmap_zalloc(unsigned int) truncation -> kvzalloc with BITS_TO_LONGS - Fix integer underflow in SET_PRESENT when start_index < pgoff - gmem_release: zap active guest mappings + clear slot->gmem.file before freeing the backing - Pin THIS_MODULE while a provider fd is open - gmem_mmap: refuse if the fd was not created mmap-capable - gmem_max_order: clamp by the absent bitmap - Bind/unbind: symmetric psmash + rmp_make_shared with proper 2MiB alignment Selftests: - Revoke test: assert the revoked access faults unconditionally - iommufd test: mark the range KVM_MEMORY_ATTRIBUTE_PRIVATE so the guest actually exercises the provider path rather than the shared HVA mmap - NVMe DMA test: read the vendor ID from PCI config and add compiler barriers before the doorbell writes Connor Williamson (1): KVM: SEV: Remove struct page dependency from SNP gmem paths David Woodhouse (10): KVM: selftests: sev_smoke_test: Only run VM types the host offers KVM: selftests: sev_init2_tests: Derive SEV availability from KVM KVM: guest_memfd: Introduce guest memory ops and route native gmem through them iommufd: Look up private-interconnect phys via exporter symbols iommufd: Plumb dma-buf memory-type (RAM vs MMIO) through the phys map KVM: guest_memfd: Add ops-driven page revocation samples/kvm: Add guest_memfd backing sample selftests/kvm: gmem_provider KVM-only tests selftests/kvm: gmem_provider iommufd tests samples/kvm, selftests/kvm: Allow the gmem_provider NVMe DMA test on arm64 arch/x86/kvm/Makefile | 5 +- arch/x86/kvm/svm/sev.c | 93 ++- arch/x86/virt/svm/sev.c | 27 +- drivers/iommu/iommufd/io_pagetable.h | 7 + drivers/iommu/iommufd/pages.c | 49 +- include/linux/kvm_host.h | 91 +++ samples/Kconfig | 18 + samples/Makefile | 1 + samples/kvm/Makefile | 2 + samples/kvm/gmem_provider.c | 772 +++++++++++++++++++++ samples/kvm/gmem_provider.h | 53 ++ tools/testing/selftests/kvm/Makefile.kvm | 6 + .../selftests/kvm/gmem_provider_nvme_dma_test.c | 376 ++++++++++ .../kvm/x86/gmem_provider_hugepage_test.c | 130 ++++ .../selftests/kvm/x86/gmem_provider_iommufd_test.c | 158 +++++ .../selftests/kvm/x86/gmem_provider_revoke_test.c | 135 ++++ .../testing/selftests/kvm/x86/gmem_provider_test.c | 195 ++++++ .../selftests/kvm/x86/gmem_provider_vfio_test.c | 134 ++++ tools/testing/selftests/kvm/x86/sev_init2_tests.c | 16 +- tools/testing/selftests/kvm/x86/sev_smoke_test.c | 9 +- virt/kvm/guest_memfd.c | 391 +++++++++-- virt/kvm/kvm_main.c | 8 +- virt/kvm/kvm_mm.h | 4 +- 23 files changed, 2577 insertions(+), 103 deletions(-)
