KVM: x86: SVM: Intercept #GP to support access to VMware backdoor ports

If KVM enable_vmware_backdoor module parameter is set,
the commit change VMX to now intercept #GP instead of being directly
deliviered from CPU to guest.

It is done to support access to VMware Backdoor I/O ports
even if TSS I/O permission denies it.
In that case:
1. A #GP will be raised and intercepted.
2. #GP intercept handler will simulate I/O port access instruction.
3. I/O port access instruction simulation will allow access to VMware
backdoor ports specifically even if TSS I/O permission bitmap denies it.

Note that the above change introduce slight performance hit as now #GPs
are now not deliviered directly from CPU to guest but instead
cause #VMExit and instruction emulation.
However, this behavior is introduced only when enable_vmware_backdoor
KVM module parameter is set.

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

arch/x86/kvm/svm.c

@@ -1364,6 +1364,14 @@ static void init_vmcb(struct vcpu_svm *svm)
 	set_exception_intercept(svm, MC_VECTOR);
 	set_exception_intercept(svm, AC_VECTOR);
 	set_exception_intercept(svm, DB_VECTOR);
+	/*
+	 * Guest access to VMware backdoor ports could legitimately
+	 * trigger #GP because of TSS I/O permission bitmap.
+	 * We intercept those #GP and allow access to them anyway
+	 * as VMware does.
+	 */
+	if (enable_vmware_backdoor)
+		set_exception_intercept(svm, GP_VECTOR);
 
 	set_intercept(svm, INTERCEPT_INTR);
 	set_intercept(svm, INTERCEPT_NMI);
@@ -2552,6 +2560,23 @@ static int ac_interception(struct vcpu_svm *svm)
 	return 1;
 }
 
+static int gp_interception(struct vcpu_svm *svm)
+{
+	struct kvm_vcpu *vcpu = &svm->vcpu;
+	u32 error_code = svm->vmcb->control.exit_info_1;
+	int er;
+
+	WARN_ON_ONCE(!enable_vmware_backdoor);
+
+	er = emulate_instruction(vcpu,
+		EMULTYPE_VMWARE | EMULTYPE_NO_UD_ON_FAIL);
+	if (er == EMULATE_USER_EXIT)
+		return 0;
+	else if (er != EMULATE_DONE)
+		kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
+	return 1;
+}
+
 static bool is_erratum_383(void)
 {
 	int err, i;
@@ -4551,6 +4576,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
 	[SVM_EXIT_EXCP_BASE + PF_VECTOR]	= pf_interception,
 	[SVM_EXIT_EXCP_BASE + MC_VECTOR]	= mc_interception,
 	[SVM_EXIT_EXCP_BASE + AC_VECTOR]	= ac_interception,
+	[SVM_EXIT_EXCP_BASE + GP_VECTOR]	= gp_interception,
 	[SVM_EXIT_INTR]				= intr_interception,
 	[SVM_EXIT_NMI]				= nmi_interception,
 	[SVM_EXIT_SMI]				= nop_on_interception,