Re: [PATCH v4 5/6] KVM: Rename and move CPUHP_AP_KVM_STARTING to ONLINE section

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 16, 2022, Chao Gao wrote:
> The CPU STARTING section doesn't allow callbacks to fail. Move KVM's
> hotplug callback to ONLINE section so that it can abort onlining a CPU in
> certain cases to avoid potentially breaking VMs running on existing CPUs.
> For example, when kvm fails to enable hardware virtualization on the
> hotplugged CPU.
> 
> Place KVM's hotplug state before CPUHP_AP_SCHED_WAIT_EMPTY as it ensures
> when offlining a CPU, all user tasks and non-pinned kernel tasks have left
> the CPU, i.e. there cannot be a vCPU task around. So, it is safe for KVM's
> CPU offline callback to disable hardware virtualization at that point.
> Likewise, KVM's online callback can enable hardware virtualization before
> any vCPU task gets a chance to run on hotplugged CPUs.
> 
> KVM's CPU hotplug callbacks are renamed as well.
> 
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---

For the KVM bits,

  Reviewed-by: Sean Christopherson <seanjc@google.com>

Someone with more knowledge of the CPU hotplug sequences should really review this
too.