[RFC PATCH 0/9] kvm: implement atomic memslot updates

Emanuele Giuseppe Esposito
2022-09-09T10:45:23Z

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@redhat.com/
"[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(-)

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2.31.1