KVM: x86: Add module parameter for supporting VMware backdoor

Support access to VMware backdoor requires KVM to intercept #GP
exceptions from guest which introduce slight performance hit.
Therefore, control this support by module parameter.

Note that module parameter is exported as it should be consumed by
kvm_intel & kvm_amd to determine if they should intercept #GP or not.

This commit doesn't change semantics.
It is done as a preparation for future commits.
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>

Documentation/admin-guide/kernel-parameters.txt

@@ -1887,6 +1887,9 @@
 	kvm.ignore_msrs=[KVM] Ignore guest accesses to unhandled MSRs.
 			Default is 0 (don't ignore, but inject #GP)
 
+	kvm.enable_vmware_backdoor=[KVM] Support VMware backdoor PV interface.
+				   Default is false (don't support).
+
 	kvm.mmu_audit=	[KVM] This is a R/W parameter which allows audit
 			KVM MMU at runtime.
 			Default is 0 (off)

arch/x86/kvm/x86.c

@@ -142,6 +142,10 @@ module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
 static bool __read_mostly vector_hashing = true;
 module_param(vector_hashing, bool, S_IRUGO);
 
+bool __read_mostly enable_vmware_backdoor = false;
+module_param(enable_vmware_backdoor, bool, S_IRUGO);
+EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
+
 #define KVM_NR_SHARED_MSRS 16
 
 struct kvm_shared_msrs_global {

arch/x86/kvm/x86.h

@@ -242,6 +242,8 @@ extern unsigned int min_timer_period_us;
 
 extern unsigned int lapic_timer_advance_ns;
 
+extern bool enable_vmware_backdoor;
+
 extern struct static_key kvm_no_apic_vcpu;
 
 static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)