Re: [RFC v2 2/2] KVM: VMX: Enable bus lock VM exit

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Sep 01, 2020 at 10:43:12AM +0200, Vitaly Kuznetsov wrote:
> > @@ -6809,6 +6824,19 @@ 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 (kvm_bus_lock_exit_enabled(vmx->vcpu.kvm)) {
> > +		if (vmx->exit_reason.bus_lock_detected) {
> > +			vcpu->stat.bus_locks++;

Why bother with stats?  Every bus lock exits to userspace, having quick
stats doesn't seem all that interesting.

> > +			vcpu->arch.bus_lock_detected = true;
> > +		} else {
> > +			vcpu->arch.bus_lock_detected = false;
> 
> This is a fast path so I'm wondering if we can move bus_lock_detected
> clearing somewhere else.

Why even snapshot vmx->exit_reason.bus_lock_detected?  I don't see any
reason why vcpu_enter_guest() needs to handle the exit to userspace, e.g.
it's just as easily handled in VMX code.

> 
> > +		}
> > +	}
> > +
> >  	vmx->loaded_vmcs->launched = 1;
> >  	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> >  
> > @@ -8060,6 +8088,9 @@ static __init int hardware_setup(void)
> >  		kvm_tsc_scaling_ratio_frac_bits = 48;
> >  	}
> >  
> > +	if (cpu_has_vmx_bus_lock_detection())
> > +		kvm_has_bus_lock_exit = true;
> > +
> >  	set_bit(0, vmx_vpid_bitmap); /* 0 is reserved for host */
> >  
> >  	if (enable_ept)

...

> > @@ -4990,6 +4996,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >  		kvm->arch.exception_payload_enabled = cap->args[0];
> >  		r = 0;
> >  		break;
> > +	case KVM_CAP_X86_BUS_LOCK_EXIT:
> > +		if (!kvm_has_bus_lock_exit)
> > +			return -EINVAL;
> 
> ... because userspace can check for -EINVAL when enabling the cap. Or we
> can return e.g. -EOPNOTSUPP here. I don't have a strong opinion on the matter..
> 
> > +		kvm->arch.bus_lock_exit = cap->args[0];

Assuming we even want to make this per-VM, I think it'd make sense to make
args[0] a bit mask, e.g. to provide "off" and "exit" (this behavior) while
allowing for future modes, e.g. log-only.

> > +		r = 0;
> > +		break;
> >  	default:
> >  		r = -EINVAL;
> >  		break;
> > @@ -7732,12 +7744,23 @@ static void post_kvm_run_save(struct kvm_vcpu *vcpu)