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>
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1d1d53f..0f5e3c8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/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)