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

[Xiaoyao Li]

On 7/1/2020 8:44 PM, Vitaly Kuznetsov wrote:
> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
>
> > 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.
>
> Exiting to userspace would allow to implement custom 'throttling'
> policies to mitigate the 'noisy neighbour' problem. The simplest would
> be to just inject some sleep time.

So you want an exit to userspace for every bus lock and leave it all to 
userspace. Yes, it's doable.

As you said, the exit_reason.bus_lock_detected case is the tricky one. 
We cannot do the similar to extend vcpu->run->exit_reason, this breaks 
ABI. Maybe we can extend the vcpu->run->flags to indicate bus lock 
detected for the other exit reason?