KVM: SVM: Treat all "skip" emulation for SEV guests as outright failures

Treat EMULTYPE_SKIP failures on SEV guests as unhandleable emulation
instead of simply resuming the guest, and drop the hack-a-fix which
effects that behavior for the INT3/INTO injection path.  If KVM can't
skip an instruction for which KVM has already done partial emulation,
resuming the guest is undesirable as doing so may corrupt guest state.

Link: https://lore.kernel.org/r/20230825013621.2845700-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/svm/svm.c

@@ -364,8 +364,6 @@ static void svm_set_interrupt_shadow(struct kvm_vcpu *vcpu, int mask)
 		svm->vmcb->control.int_state |= SVM_INTERRUPT_SHADOW_MASK;
 
 }
-static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
-					 void *insn, int insn_len);
 
 static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
 					   bool commit_side_effects)
@@ -386,14 +384,6 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
 	}
 
 	if (!svm->next_rip) {
-		/*
-		 * FIXME: Drop this when kvm_emulate_instruction() does the
-		 * right thing and treats "can't emulate" as outright failure
-		 * for EMULTYPE_SKIP.
-		 */
-		if (svm_check_emulate_instruction(vcpu, EMULTYPE_SKIP, NULL, 0) != X86EMUL_CONTINUE)
-			return 0;
-
 		if (unlikely(!commit_side_effects))
 			old_rflags = svm->vmcb->save.rflags;
 
@@ -4781,7 +4771,7 @@ static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
 	 */
 	if (unlikely(!insn)) {
 		if (emul_type & EMULTYPE_SKIP)
-			return X86EMUL_RETRY_INSTR;
+			return X86EMUL_UNHANDLEABLE;
 
 		kvm_queue_exception(vcpu, UD_VECTOR);
 		return X86EMUL_PROPAGATE_FAULT;

arch/x86/kvm/x86.c

@@ -8874,8 +8874,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 	bool writeback = true;
 
 	r = kvm_check_emulate_insn(vcpu, emulation_type, insn, insn_len);
-	if (r != X86EMUL_CONTINUE)
-		return 1;
+	if (r != X86EMUL_CONTINUE) {
+		if (r == X86EMUL_RETRY_INSTR || r == X86EMUL_PROPAGATE_FAULT)
+			return 1;
+
+		WARN_ON_ONCE(r != X86EMUL_UNHANDLEABLE);
+		return handle_emulation_failure(vcpu, emulation_type);
+	}
 
 	vcpu->arch.l1tf_flush_l1d = true;