[PATCH 0/4] KVM: API to block and resume all running vcpus in a vm

Emanuele Giuseppe Esposito
2022-10-22T15:48:33Z

This new API allows the userspace to stop all running
vcpus using KVM_KICK_ALL_RUNNING_VCPUS ioctl, and resume them with
KVM_RESUME_ALL_KICKED_VCPUS.
A "running" vcpu is a vcpu that is executing the KVM_RUN ioctl.

This serie is especially helpful to userspace hypervisors like
QEMU when they need to perform operations on memslots without the
risk of having a vcpu reading them in the meanwhile.
With "memslots operations" we mean grow, shrink, merge and split
memslots, which are not "atomic" because there is a time window
between the DELETE memslot operation and the CREATE one.
Currently, each memslot operation is performed with one or more
ioctls.
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.

Therefore the simplest solution is to pause all vcpus in the kvm
side, so that:
- userspace 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

Emanuele Giuseppe Esposito (4):
linux-headers/linux/kvm.h: introduce kvm_userspace_memory_region_list
ioctl
KVM: introduce kvm_clear_all_cpus_request
KVM: introduce memory transaction semaphore
KVM: use signals to abort enter_guest/blocking and retry

Documentation/virt/kvm/vcpu-requests.rst | 3 ++
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/x86.c | 8 +++++
include/uapi/linux/kvm.h | 3 ++
virt/kvm/kvm_main.c | 45 ++++++++++++++++++++++++
5 files changed, 61 insertions(+)

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2.31.1