Re: [PATCH 2/3] KVM: x86: move kvm_inject_gp up from kvm_handle_invpcid to callers

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 02, 2021, Paolo Bonzini wrote:
> Push the injection of #GP up to the callers, so that they can just use
> kvm_complete_insn_gp.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

...

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 08568c47337c..edbeb162012b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11375,7 +11375,6 @@ int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva)
>  		return kvm_handle_memory_failure(vcpu, r, &e);

This is broken.  kvm_handle_memory_failure() injects a #PF and returns 1, which
means an error here will trigger #PF->#GP->#DF.

>  
>  	if (operand.pcid >> 12 != 0) {
> -		kvm_inject_gp(vcpu, 0);
>  		return 1;
>  	}

Curly braces can be dropped.

> @@ -11385,15 +11384,13 @@ int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva)
>  	case INVPCID_TYPE_INDIV_ADDR:
>  		if ((!pcid_enabled && (operand.pcid != 0)) ||
>  		    is_noncanonical_address(operand.gla, vcpu)) {
> -			kvm_inject_gp(vcpu, 0);
>  			return 1;
>  		}
>  		kvm_mmu_invpcid_gva(vcpu, operand.gla, operand.pcid);
> -		return kvm_skip_emulated_instruction(vcpu);
> +		return 0;
>  
>  	case INVPCID_TYPE_SINGLE_CTXT:
>  		if (!pcid_enabled && (operand.pcid != 0)) {
> -			kvm_inject_gp(vcpu, 0);
>  			return 1;
>  		}

Same here.

>  
> @@ -11414,7 +11411,7 @@ int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva)
>  		 * resync will happen anyway before switching to any other CR3.
>  		 */
>  
> -		return kvm_skip_emulated_instruction(vcpu);
> +		return 0;
>  
>  	case INVPCID_TYPE_ALL_NON_GLOBAL:
>  		/*
> @@ -11427,7 +11424,7 @@ int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva)
>  		fallthrough;
>  	case INVPCID_TYPE_ALL_INCL_GLOBAL:
>  		kvm_mmu_unload(vcpu);
> -		return kvm_skip_emulated_instruction(vcpu);
> +		return 0;
>  
>  	default:
>  		BUG(); /* We have already checked above that type <= 3 */
> -- 
> 2.26.2

IMO, this isn't an improvement.  For flows that can't easily be consolidated to
x86.c, e.g. CRs (and DRs?), I agree it makes sense to use kvm_complete_insn_gp(),
but this feels forced.

What about a pure refactoring of kvm_handle_invpcid() to get a similar effect?

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ef630f8d8bd2..b9f57f9f2422 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11363,28 +11363,23 @@ int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva)
        if (r != X86EMUL_CONTINUE)
                return kvm_handle_memory_failure(vcpu, r, &e);
 
-       if (operand.pcid >> 12 != 0) {
-               kvm_inject_gp(vcpu, 0);
-               return 1;
-       }
+       if (operand.pcid >> 12 != 0)
+               goto inject_gp;
 
        pcid_enabled = kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE);
 
        switch (type) {
        case INVPCID_TYPE_INDIV_ADDR:
                if ((!pcid_enabled && (operand.pcid != 0)) ||
-                   is_noncanonical_address(operand.gla, vcpu)) {
-                       kvm_inject_gp(vcpu, 0);
-                       return 1;
-               }
+                   is_noncanonical_address(operand.gla, vcpu))
+                       goto inject_gp;
+
                kvm_mmu_invpcid_gva(vcpu, operand.gla, operand.pcid);
-               return kvm_skip_emulated_instruction(vcpu);
+               break;
 
        case INVPCID_TYPE_SINGLE_CTXT:
-               if (!pcid_enabled && (operand.pcid != 0)) {
-                       kvm_inject_gp(vcpu, 0);
-                       return 1;
-               }
+               if (!pcid_enabled && (operand.pcid != 0))
+                       goto inject_gp;
 
                if (kvm_get_active_pcid(vcpu) == operand.pcid) {
                        kvm_mmu_sync_roots(vcpu);
@@ -11402,8 +11397,7 @@ int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva)
                 * given PCID, then nothing needs to be done here because a
                 * resync will happen anyway before switching to any other CR3.
                 */
-
-               return kvm_skip_emulated_instruction(vcpu);
+               break;
 
        case INVPCID_TYPE_ALL_NON_GLOBAL:
                /*
@@ -11416,11 +11410,17 @@ int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva)
                fallthrough;
        case INVPCID_TYPE_ALL_INCL_GLOBAL:
                kvm_mmu_unload(vcpu);
-               return kvm_skip_emulated_instruction(vcpu);
+               break;
 
        default:
                BUG(); /* We have already checked above that type <= 3 */
        }
+
+       return kvm_skip_emulated_instruction(vcpu);
+
+inject_gp:
+       kvm_inject_gp(vcpu, 0);
+       return 1;
 }
 EXPORT_SYMBOL_GPL(kvm_handle_invpcid);