Re: [PATCH 10/14] KVM: x86: Make Hyper-V emulation optional

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 30, 2023, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> ...
> 
> >
> >>  static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> >>  {
> >> @@ -3552,11 +3563,13 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
> >>  	if (!nested_vmx_check_permission(vcpu))
> >>  		return 1;
> >>  
> >> +#ifdef CONFIG_KVM_HYPERV
> >>  	evmptrld_status = nested_vmx_handle_enlightened_vmptrld(vcpu, launch);
> >>  	if (evmptrld_status == EVMPTRLD_ERROR) {
> >>  		kvm_queue_exception(vcpu, UD_VECTOR);
> >>  		return 1;
> >>  	}
> >> +#endif
> >>  
> >>  	kvm_pmu_trigger_event(vcpu, PERF_COUNT_HW_BRANCH_INSTRUCTIONS);
> >
> > This fails to build with CONFIG_KVM_HYPERV=n && CONFIG_KVM_WERROR=y:
> >
> > arch/x86/kvm/vmx/nested.c:3577:9: error: variable 'evmptrld_status' is uninitialized when used here [-Werror,-Wuninitialized]
> >         if (CC(evmptrld_status == EVMPTRLD_VMFAIL))
> >                ^~~~~~~~~~~~~~~
> >
> > Sadly, simply wrapping with an #ifdef also fails because then evmptrld_status
> > becomes unused.  I'd really prefer to avoid having to tag it __maybe_unused, and
> > adding more #ifdef would also be ugly.  Any ideas?
> 
> A couple,
> 
> - we can try putting all eVMPTR logic under 'if (1)' just to create a
>   block where we can define evmptrld_status. Not sure this is nicer than
>   another #ifdef wrapping evmptrld_status and I'm not sure what to do
>   with kvm_pmu_trigger_event() -- can it just go above
>   nested_vmx_handle_enlightened_vmptrld()?
> 
> - we can add a helper, e.g. 'evmptr_is_vmfail()' and make it just return
>   'false' when !CONFIG_KVM_HYPERV.
> 
> - rewrite this as a switch to avoid the need for having the local
>   variable, (untested)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index c5ec0ef51ff7..b26ce7d596e9 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3553,22 +3553,23 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
>         enum nvmx_vmentry_status status;
>         struct vcpu_vmx *vmx = to_vmx(vcpu);
>         u32 interrupt_shadow = vmx_get_interrupt_shadow(vcpu);
> -       enum nested_evmptrld_status evmptrld_status;
>  
>         if (!nested_vmx_check_permission(vcpu))
>                 return 1;
>  
> -       evmptrld_status = nested_vmx_handle_enlightened_vmptrld(vcpu, launch);
> -       if (evmptrld_status == EVMPTRLD_ERROR) {
> +       switch (nested_vmx_handle_enlightened_vmptrld(vcpu, launch)) {
> +       case EVMPTRLD_ERROR:
>                 kvm_queue_exception(vcpu, UD_VECTOR);
>                 return 1;
> +       case EVMPTRLD_VMFAIL:
> +               trace_kvm_nested_vmenter_failed("evmptrld_status", 0);
> +               return nested_vmx_failInvalid(vcpu);
> +       default:
> +               break;
>         }
>  
>         kvm_pmu_trigger_event(vcpu, PERF_COUNT_HW_BRANCH_INSTRUCTIONS);
>  
> -       if (CC(evmptrld_status == EVMPTRLD_VMFAIL))
> -               return nested_vmx_failInvalid(vcpu);
> -
>         if (CC(!evmptr_is_valid(vmx->nested.hv_evmcs_vmptr) &&
>                vmx->nested.current_vmptr == INVALID_GPA))
>                 return nested_vmx_failInvalid(vcpu);
> 
> Unfortunately, I had to open code CC() ;-(
> 
> Or maybe just another "#ifdef" is not so ugly after all? :-)

Ah, just have nested_vmx_handle_enlightened_vmptrld() return EVMPTRLD_DISABLED
for its "stub", e.g. with some otehr tangentially de-stubbing:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 4777d867419c..5a27a2ebbb32 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2000,6 +2000,7 @@ static void copy_vmcs12_to_enlightened(struct vcpu_vmx *vmx)
 static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
        struct kvm_vcpu *vcpu, bool from_launch)
 {
+#ifdef CONFIG_KVM_HYPERV
        struct vcpu_vmx *vmx = to_vmx(vcpu);
        bool evmcs_gpa_changed = false;
        u64 evmcs_gpa;
@@ -2081,11 +2082,10 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
        }
 
        return EVMPTRLD_SUCCEEDED;
+#else
+       return EVMPTRLD_DISABLED;
+#endif
 }
-#else /* CONFIG_KVM_HYPERV */
-static inline void copy_enlightened_to_vmcs12(struct vcpu_vmx *vmx, u32 hv_clean_fields) {}
-static inline void copy_vmcs12_to_enlightened(struct vcpu_vmx *vmx) {}
-#endif /* CONFIG_KVM_HYPERV */
 
 void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
 {
@@ -3191,8 +3191,6 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
 
        return true;
 }
-#else
-static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu) { return true; }
 #endif
 
 static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
@@ -3285,6 +3283,7 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
 
 static bool vmx_get_nested_state_pages(struct kvm_vcpu *vcpu)
 {
+#ifdef CONFIG_KVM_HYPERV
        /*
         * Note: nested_get_evmcs_page() also updates 'vp_assist_page' copy
         * in 'struct kvm_vcpu_hv' in case eVMCS is in use, this is mandatory
@@ -3301,7 +3300,7 @@ static bool vmx_get_nested_state_pages(struct kvm_vcpu *vcpu)
 
                return false;
        }
-
+#endif
        if (is_guest_mode(vcpu) && !nested_get_vmcs12_pages(vcpu))
                return false;
 
@@ -3564,7 +3563,6 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
        if (!nested_vmx_check_permission(vcpu))
                return 1;
 
-#ifdef CONFIG_KVM_HYPERV
        evmptrld_status = nested_vmx_handle_enlightened_vmptrld(vcpu, launch);
        if (evmptrld_status == EVMPTRLD_ERROR) {
                kvm_queue_exception(vcpu, UD_VECTOR);
@@ -4743,6 +4741,7 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
        WARN_ON_ONCE(vmx->nested.nested_run_pending);
 
        if (kvm_check_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu)) {
+#ifdef CONFIG_KVM_HYPERV
                /*
                 * KVM_REQ_GET_NESTED_STATE_PAGES is also used to map
                 * Enlightened VMCS after migration and we still need to
@@ -4750,6 +4749,7 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
                 * the first L2 run.
                 */
                (void)nested_get_evmcs_page(vcpu);
+#endif
        }
 
        /* Service pending TLB flush requests for L2 before switching to L1. */