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

Sean Christopherson
2022-09-09T14:30:56Z

On Fri, Sep 09, 2022, Emanuele Giuseppe Esposito wrote:
> 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

I don't have access. Can you provide a TL;DR?

> 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.

Why not simply pause vCPUs in this scenario? This is an awful lot of a complexity
to take on for something that appears to be solvable in userspace.

And if the issue is related to KVM disallowing the toggling of read-only (can't see
the bug), we can likely solve that without needing a new ioctl() that allows
userspace to batch an arbitrary number of updates.