Am 22.10.22 um 17:48 schrieb Emanuele Giuseppe Esposito:
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
Isnt QEMU able to achieve the same goal today by forcing all vCPUs into userspace with a signal? Can you provide some rationale why this is better in the cover letter or patch description?
