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 spend 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 stateallows 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 that 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 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 | 4 +- arch/x86/kvm/svm/sev.c | 83 ++- 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 | 651 +++++++++++++++++++++ samples/kvm/gmem_provider.h | 53 ++ tools/testing/selftests/kvm/Makefile.kvm | 6 + .../selftests/kvm/gmem_provider_nvme_dma_test.c | 365 ++++++++++++ .../kvm/x86/gmem_provider_hugepage_test.c | 130 ++++ .../selftests/kvm/x86/gmem_provider_iommufd_test.c | 150 +++++ .../selftests/kvm/x86/gmem_provider_revoke_test.c | 134 +++++ .../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 | 377 +++++++++--- virt/kvm/kvm_main.c | 6 +- virt/kvm/kvm_mm.h | 4 +- 23 files changed, 2410 insertions(+), 102 deletions(-)
