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

[Christian Borntraeger]

Am 24.10.22 um 10:33 schrieb Emanuele Giuseppe Esposito:
> Am 24/10/2022 um 09:56 schrieb Christian Borntraeger:
> > 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?
> David Hildenbrand tried to propose something similar here:
> https://github.com/davidhildenbrand/qemu/commit/86b1bf546a8d00908e33f7362b0b61e2be8dbb7a
>
> While it is not optimized, I think it's more complex that the current
> serie, since qemu should also make sure all running ioctls finish and
> prevent the new ones from getting executed.
>
> Also we can't use pause_all_vcpus()/resume_all_vcpus() because they drop
> the BQL.
>
> Would that be ok as rationale?

Yes that helps and should be part of the cover letter for the next iterations.