Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit

[Xiaoyao Li]

On 7/1/2020 5:04 PM, Vitaly Kuznetsov wrote:
> Chenyi Qiang <chenyi.qiang@xxxxxxxxx> writes:
[...]
> >   static const int kvm_vmx_max_exit_handlers =
> > @@ -6830,6 +6838,13 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >   	if (unlikely(vmx->exit_reason.failed_vmentry))
> >   		return EXIT_FASTPATH_NONE;
> >   
> > +	/*
> > +	 * check the exit_reason to see if there is a bus lock
> > +	 * happened in guest.
> > +	 */
> > +	if (vmx->exit_reason.bus_lock_detected)
> > +		handle_bus_lock(vcpu);
>
> In case the ultimate goal is to have an exit to userspace on bus lock,

I don't think we will need an exit to userspace on bus lock. See below.

> the two ways to reach handle_bus_lock() are very different: in case
> we're handling EXIT_REASON_BUS_LOCK we can easily drop to userspace by
> returning 0 but what are we going to do in case of
> exit_reason.bus_lock_detected? The 'higher priority VM exit' may require
> exit to userspace too. So what's the plan? Maybe we can ignore the case
> when we're exiting to userspace for some other reason as this is slow
> already and force the exit otherwise? 

> And should we actually introduce
> the KVM_EXIT_BUS_LOCK and a capability to enable it here?

Introducing KVM_EXIT_BUS_LOCK maybe help nothing. No matter 
EXIT_REASON_BUS_LOCK or exit_reason.bus_lock_detected, the bus lock has 
already happened. Exit to userspace cannot prevent bus lock, so what 
userspace can do is recording and counting as what this patch does in 
vcpu->stat.bus_locks.