KVM: VMX: Skip global INVVPID fallback if vpid==0 in vpid_sync_context()

Skip the global INVVPID in the unlikely scenario that vpid==0 and the
SINGLE_CONTEXT variant of INVVPID is unsupported.  If vpid==0, there's
no need to INVVPID as it's impossible to do VM-Enter with VPID enabled
and vmcs.VPID==0, i.e. there can't be any TLB entries for the vCPU with
vpid==0.  The fact that the SINGLE_CONTEXT variant isn't supported is
irrelevant.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320212833.3507-9-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/ops.h

@@ -300,7 +300,7 @@ static inline void vpid_sync_context(int vpid)
 {
 	if (cpu_has_vmx_invvpid_single())
 		vpid_sync_vcpu_single(vpid);
-	else
+	else if (vpid != 0)
 		vpid_sync_vcpu_global();
 }