[0/2] KVM: stop all vcpus before modifying memslots

Message

Emanuele Giuseppe Esposito Oct. 22, 2022, 3:48 p.m. UTC

QEMU needs to perform memslots operations like merging and splitting,
and each operation requires more than a single ioctl.
Therefore if a vcpu is concurrently reading the same memslots,
it could end up reading something that was temporarly deleted.
For example, merging two memslots into one would imply:
DELETE(m1)
DELETE(m2)
CREATE(m1+m2)

And a vcpu could attempt to read m2 right after it is deleted, but
before the new one is created.

To solve this problem, use the newly introduced kvm API:
KVM_KICK_ALL_RUNNING_VCPUS and KVM_RESUME_ALL_KICKED_VCPUS.
This new API allows the userspace to respectively stop and resume all running vcpus. A "running" vcpu is a vcpu that is executing
the KVM_RUN ioctl.

While KVM already handles the case of KVM_RUN being called after
KVM_KICK_ALL_RUNNING_VCPUS is invoked but before KVM_RESUME_ALL_KICKED_VCPUS by simply returning immediately,
QEMU also avoids that using the event API.

This is the simplest solution, pausing all vcpus in the kvm
side, so that:
- QEMU just needs to call the new API before making memslots
changes, keeping modifications to the minimum
- dirty page updates are also performed when vcpus are blocked, so
there is no time window between the dirty page ioctl and memslots
modifications, since vcpus are all stopped.
- no need to modify the existing memslots API

This series requires the KVM serie "KVM: API to block and resume all running vcpus in a vm".

Emanuele Giuseppe Esposito (2):
  linux-headers/linux/kvm.h: introduce kvm_userspace_memory_region_list
    ioctl
  accel/kvm: introduce begin/commit listener callbacks

 accel/kvm/kvm-all.c       | 50 +++++++++++++++++++++++++++++++++++++++
 linux-headers/linux/kvm.h |  3 +++
 2 files changed, 53 insertions(+)