KVM: SVM: add module param to control the #SMI interception

In theory there are no side effects of not intercepting #SMI,
because then #SMI becomes transparent to the OS and the KVM.

Plus an observation on recent Zen2 CPUs reveals that these
CPUs ignore #SMI interception and never deliver #SMI VMexits.

This is also useful to test nested KVM to see that L1
handles #SMIs correctly in case when L1 doesn't intercept #SMI.

Finally the default remains the same, the SMI are intercepted
by default thus this patch doesn't have any effect unless
non default module param value is used.
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210707125100.677203-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/nested.c

@@ -154,6 +154,10 @@ void recalc_intercepts(struct vcpu_svm *svm)
 
 	for (i = 0; i < MAX_INTERCEPT; i++)
 		c->intercepts[i] |= g->intercepts[i];
+
+	/* If SMI is not intercepted, ignore guest SMI intercept as well  */
+	if (!intercept_smi)
+		vmcb_clr_intercept(c, INTERCEPT_SMI);
 }
 
 static void copy_vmcb_control_area(struct vmcb_control_area *dst,

arch/x86/kvm/svm/svm.c

@@ -198,6 +198,11 @@ module_param(avic, bool, 0444);
 bool __read_mostly dump_invalid_vmcb;
 module_param(dump_invalid_vmcb, bool, 0644);
 
+
+bool intercept_smi = true;
+module_param(intercept_smi, bool, 0444);
+
+
 static bool svm_gp_erratum_intercept = true;
 
 static u8 rsm_ins_bytes[] = "\x0f\xaa";
@@ -1185,7 +1190,10 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
 
 	svm_set_intercept(svm, INTERCEPT_INTR);
 	svm_set_intercept(svm, INTERCEPT_NMI);
+
+	if (intercept_smi)
 		svm_set_intercept(svm, INTERCEPT_SMI);
+
 	svm_set_intercept(svm, INTERCEPT_SELECTIVE_CR0);
 	svm_set_intercept(svm, INTERCEPT_RDPMC);
 	svm_set_intercept(svm, INTERCEPT_CPUID);

arch/x86/kvm/svm/svm.h

@@ -31,6 +31,7 @@
 #define MSRPM_OFFSETS	16
 extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
 extern bool npt_enabled;
+extern bool intercept_smi;
 
 /*
  * Clean bits in VMCB.