In preparation for adding more features to KVM's guest_memfd, refactor and introduce a library which abstracts some of the core-mm decisions about managing folios associated with the file. The goal of the refactor serves two purposes: 1. Provide an easier way to reason about memory in guest_memfd. With KVM supporting multiple confidentiality models (TDX, SEV-SNP, pKVM, ARM CCA), and coming support for allowing kernel and userspace to access this memory, it seems necessary to create a stronger abstraction between core-mm concerns and hypervisor concerns. 2. Provide a common implementation for other hypervisors (Gunyah) to use. To create a guest_memfd, the owner provides operations to attempt to unmap the folio and check whether a folio is accessible to the host. The owner can call guest_memfd_make_inaccessible() to ensure Linux doesn't have the folio mapped. The series first introduces a guest_memfd library based on the current KVM (next) implementation, then adds few features needed for Gunyah and arm64 pKVM. The Gunyah usage of the series will be posted separately shortly after sending this series. I'll work with Fuad on using the guest_memfd library for arm64 pKVM based on the feedback received. There are a few TODOs still pending. - The KVM patch isn't tested. I don't have access a SEV-SNP setup to be able to test. - I've not yet investigated deeply whether having the guest_memfd library helps live migration. I'd appreciate any input on that part. - We should consider consolidating the adjust_direct_map() in arch/x86/virt/svm/sev.c so guest_memfd can take care of it. - There's a race possibility where the folio ref count is incremented and about to also increment the safe counter, but waiting for the folio lock to be released. The owner of folio_lock will see mismatched counter values and not be able to convert to (in)accessible, even though it should be okay to do so. I'd appreciate any feedback, especially on the direction I'm taking for tracking the (in)accessible state. Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx> Changes in v2: - Significantly reworked to introduce "accessible" and "safe" reference counters - Link to v1: https://lore.kernel.org/r/20240805-guest-memfd-lib-v1-0-e5a29a4ff5d7@xxxxxxxxxxx --- Elliot Berman (5): mm: Introduce guest_memfd mm: guest_memfd: Allow folios to be accessible to host kvm: Convert to use guest_memfd library mm: guest_memfd: Add ability for userspace to mmap pages mm: guest_memfd: Add option to remove inaccessible memory from direct map arch/x86/kvm/svm/sev.c | 3 +- include/linux/guest_memfd.h | 49 ++++ mm/Kconfig | 3 + mm/Makefile | 1 + mm/guest_memfd.c | 667 ++++++++++++++++++++++++++++++++++++++++++++ virt/kvm/Kconfig | 1 + virt/kvm/guest_memfd.c | 371 +++++------------------- virt/kvm/kvm_main.c | 2 - virt/kvm/kvm_mm.h | 6 - 9 files changed, 797 insertions(+), 306 deletions(-) --- base-commit: 5be63fc19fcaa4c236b307420483578a56986a37 change-id: 20240722-guest-memfd-lib-455f24115d46 Best regards, -- Elliot Berman <quic_eberman@xxxxxxxxxxx>
