KVM: x86: Default to not allowing emulation retry in kvm_mmu_page_fault

Effectively force kvm_mmu_page_fault() to opt-in to allowing retry to
make it more obvious when and why it allows emulation to be retried.
Previously this approach was less convenient due to retry and
re-execute behavior being controlled by separate flags that were also
inverted in their implementations (opt-in versus opt-out).

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/mmu.c

@@ -5217,7 +5217,7 @@ static int make_mmu_pages_available(struct kvm_vcpu *vcpu)
 int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
 		       void *insn, int insn_len)
 {
-	int r, emulation_type = EMULTYPE_ALLOW_RETRY;
+	int r, emulation_type = 0;
 	enum emulation_result er;
 	bool direct = vcpu->arch.mmu.direct_map;
 
@@ -5230,10 +5230,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
 	r = RET_PF_INVALID;
 	if (unlikely(error_code & PFERR_RSVD_MASK)) {
 		r = handle_mmio_page_fault(vcpu, cr2, direct);
-		if (r == RET_PF_EMULATE) {
-			emulation_type = 0;
+		if (r == RET_PF_EMULATE)
 			goto emulate;
-		}
 	}
 
 	if (r == RET_PF_INVALID) {
@@ -5260,8 +5258,16 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
 		return 1;
 	}
 
-	if (mmio_info_in_cache(vcpu, cr2, direct))
-		emulation_type = 0;
+	/*
+	 * vcpu->arch.mmu.page_fault returned RET_PF_EMULATE, but we can still
+	 * optimistically try to just unprotect the page and let the processor
+	 * re-execute the instruction that caused the page fault.  Do not allow
+	 * retrying MMIO emulation, as it's not only pointless but could also
+	 * cause us to enter an infinite loop because the processor will keep
+	 * faulting on the non-existent MMIO address.
+	 */
+	if (!mmio_info_in_cache(vcpu, cr2, direct))
+		emulation_type = EMULTYPE_ALLOW_RETRY;
 emulate:
 	/*
 	 * On AMD platforms, under certain conditions insn_len may be zero on #NPF.