> From: Paolo Bonzini <paolo.bonzini@xxxxxxxxx>
> Sent: Tuesday, November 30, 2021 5:29 PM
>
> On 11/30/21 09:27, Tian, Kevin wrote:
> > r = kvm_arch_hardware_enable();
> >
> > if (r) {
> > cpumask_clear_cpu(cpu, cpus_hardware_enabled);
> > atomic_inc(&hardware_enable_failed);
> > pr_info("kvm: enabling virtualization on CPU%d
> failed\n", cpu);
> > }
> > }
> >
> > Upon error hardware_enable_failed is incremented. However this variable
> > is checked only in hardware_enable_all() called when the 1st VM is called.
> >
> > This implies that KVM may be left in a state where it doesn't know a CPU
> > not ready to host VMX operations.
> >
> > Then I'm curious what will happen if a vCPU is scheduled to this CPU. Does
> > KVM indirectly catch it (e.g. vmenter fail) and return a deterministic error
> > to Qemu at some point or may it lead to undefined behavior? And is there
> > any method to prevent vCPU thread from being scheduled to the CPU?
>
> It should fail the first vmptrld instruction. It will result in a few
> WARN_ONCE and pr_warn_ratelimited (see vmx_insn_failed). For VMX this
> should be a pretty bad firmware bug, and it has never been reported.
> KVM did find some undocumented errata but not this one!
>
or it may be caused by incompatible CPU capabilities, which is currently
missing a check in kvm_starting_cpu(). So far the compatibility check is
done only once before registering cpu hotplug state machine:
for_each_online_cpu(cpu) {
smp_call_function_single(cpu, check_processor_compat, &c, 1);
if (r < 0)
goto out_free_2;
}
r = cpuhp_setup_state_nocalls(CPUHP_AP_KVM_STARTING, "kvm/cpu:starting",
kvm_starting_cpu, kvm_dying_cpu);
Thanks
Kevin
