KVM: x86: Add emulation_type to not raise #UD on emulation failure

Next commits are going introduce support for accessing VMware backdoor
ports even though guest's TSS I/O permissions bitmap doesn't allow
access. This mimic VMware hypervisor behavior.

In order to support this, next commits will change VMX/SVM to
intercept #GP which was raised by such access and handle it by calling
the x86 emulator to emulate instruction. Since commit "KVM: x86:
Always allow access to VMware backdoor I/O ports", the x86 emulator
handles access to these I/O ports by not checking these ports against
the TSS I/O permission bitmap.

However, there could be cases that CPU rasies a #GP on instruction
that fails to be disassembled by the x86 emulator (Because of
incomplete implementation for example).

In those cases, we would like the #GP intercept to just forward #GP
as-is to guest as if there was no intercept to begin with.
However, current emulator code always queues #UD exception in case
emulator fails (including disassembly failures) which is not what is
wanted in this flow.

This commit addresses this issue by adding a new emulation_type flag
that will allow the #GP intercept handler to specify that it wishes
to be aware when instruction emulation fails and doesn't want #UD
exception to be queued.

Signed-off-by: Liran Alon <liran.alon@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1193,6 +1193,7 @@ enum emulation_result {
 #define EMULTYPE_SKIP		    (1 << 2)
 #define EMULTYPE_RETRY		    (1 << 3)
 #define EMULTYPE_NO_REEXECUTE	    (1 << 4)
+#define EMULTYPE_NO_UD_ON_FAIL	    (1 << 5)
 int x86_emulate_instruction(struct kvm_vcpu *vcpu, unsigned long cr2,
 			    int emulation_type, void *insn, int insn_len);
 

arch/x86/kvm/x86.c

@@ -5670,18 +5670,23 @@ int kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip)
 }
 EXPORT_SYMBOL_GPL(kvm_inject_realmode_interrupt);
 
-static int handle_emulation_failure(struct kvm_vcpu *vcpu)
+static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
 {
 	int r = EMULATE_DONE;
 
 	++vcpu->stat.insn_emulation_fail;
 	trace_kvm_emulate_insn_failed(vcpu);
+
+	if (emulation_type & EMULTYPE_NO_UD_ON_FAIL)
+		return EMULATE_FAIL;
+
 	if (!is_guest_mode(vcpu) && kvm_x86_ops->get_cpl(vcpu) == 0) {
 		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
 		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
 		vcpu->run->internal.ndata = 0;
 		r = EMULATE_USER_EXIT;
 	}
+
 	kvm_queue_exception(vcpu, UD_VECTOR);
 
 	return r;
@@ -5977,7 +5982,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 				return EMULATE_DONE;
 			if (emulation_type & EMULTYPE_SKIP)
 				return EMULATE_FAIL;
-			return handle_emulation_failure(vcpu);
+			return handle_emulation_failure(vcpu, emulation_type);
 		}
 	}
 
@@ -6012,7 +6017,7 @@ restart:
 					emulation_type))
 			return EMULATE_DONE;
 
-		return handle_emulation_failure(vcpu);
+		return handle_emulation_failure(vcpu, emulation_type);
 	}
 
 	if (ctxt->have_exception) {