KVM is currently capable of receiving a single memslot update through the KVM_SET_USER_MEMORY_REGION ioctl. The problem arises when we want to atomically perform multiple updates, so that readers of memslot active list avoid seeing incomplete states. For example, in RHBZ https://bugzilla.redhat.com/show_bug.cgi?id=1979276 we see how non atomic updates cause boot failure, because vcpus will se a partial update (old memslot delete, new one not yet created) and will crash. In this series we introduce KVM_SET_USER_MEMORY_REGION_LIST, a new ioctl that takes a kvm_userspace_memory_region_list, a list of memslot updates, and performs them atomically. "atomically" in KVM words just means "apply all modifications to the inactive memslot list, and then perform a single swap to replace the active list with the inactive". It is slightly more complicated that that, since DELETE and MOVE operations require 2 swaps, but the main idea is the above. Patch 1-6 are just code movements, in preparation for the following patches. Patch 7 allows the invalid slot to be in both inactive and active memslot lists. Patch 8 allows searching for the existing memslot (old) in the inactive list, and not the active, allowing to perform multiple memslot updates without swapping. Patch 9 implements IOCTL logic. QEMU userspace logic in preparation for the IOCTL is here: https://patchew.org/QEMU/20220909081150.709060-1-eesposit@xxxxxxxxxx/ "[RFC PATCH v2 0/3] accel/kvm: extend kvm memory listener to support" TODOs and ideas: - limit the size of the ioctl arguments. Right now it is unbounded - try to reduce the amount of swaps necessary? ie every DELETE/MOVE requires an additional swap - add selftests - add documentation Emanuele Giuseppe Esposito (9): kvm_main.c: move slot check in kvm_set_memory_region kvm.h: introduce KVM_SET_USER_MEMORY_REGION_LIST ioctl kvm_main.c: introduce kvm_internal_memory_region_list kvm_main.c: split logic in kvm_set_memslots kvm_main.c: split __kvm_set_memory_region logic in kvm_check_mem and kvm_prepare_batch kvm_main.c: simplify change-specific callbacks kvm_main.c: duplicate invalid memslot also in inactive list kvm_main.c: find memslots from the inactive memslot list kvm_main.c: handle atomic memslot update arch/x86/kvm/x86.c | 3 +- include/linux/kvm_host.h | 21 +- include/uapi/linux/kvm.h | 21 +- virt/kvm/kvm_main.c | 420 +++++++++++++++++++++++++++++++-------- 4 files changed, 377 insertions(+), 88 deletions(-) -- 2.31.1
