Re: [PATCH v6 8/8] kvm: vmx: virtualize split lock detection

[Xiaoyao Li]

On 3/26/2020 7:10 PM, Thomas Gleixner wrote:
> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> > On 3/25/2020 8:40 AM, Thomas Gleixner wrote:
> > > Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> > > >    static int handle_exception_nmi(struct kvm_vcpu *vcpu)
> > > >    {
> > > >    	struct vcpu_vmx *vmx = to_vmx(vcpu);
> > > > @@ -4725,12 +4746,13 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
> > > >    	case AC_VECTOR:
> > > >    		/*
> > > >    		 * Reflect #AC to the guest if it's expecting the #AC, i.e. has
> > > > -		 * legacy alignment check enabled.  Pre-check host split lock
> > > > -		 * support to avoid the VMREADs needed to check legacy #AC,
> > > > -		 * i.e. reflect the #AC if the only possible source is legacy
> > > > -		 * alignment checks.
> > > > +		 * legacy alignment check enabled or split lock detect enabled.
> > > > +		 * Pre-check host split lock support to avoid further check of
> > > > +		 * guest, i.e. reflect the #AC if host doesn't enable split lock
> > > > +		 * detection.
> > > >    		 */
> > > >    		if (!split_lock_detect_on() ||
> > > > +		    guest_cpu_split_lock_detect_on(vmx) ||
> > > >    		    guest_cpu_alignment_check_enabled(vcpu)) {
> > >
> > > If the host has split lock detection disabled then how is the guest
> > > supposed to have it enabled in the first place?
> > It is ||
>
> Again. If the host has it disabled, then the feature flag is OFF. So
> how is the hypervisor exposing it in the first place?

So what's wrong with above code?

If the host has it disabled, !split_lock_detect_on() is true, it skips 
following check due to ||

I guess you want something like below?

if (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK)) {
	inject #AC back to guest
} else {
	if (guest_alignment_check_enabled() || guest_sld_on())
		inject #AC back to guest
}

BTW, there is an issue in my original patch that guest_sld_on() should 
be checked at last.