KVM: VMX: Update PT MSR intercepts during filter change iff PT in host+guest

Update the Processor Trace (PT) MSR intercepts during a filter change if
and only if PT may be exposed to the guest, i.e. only if KVM is operating
in the so called "host+guest" mode where PT can be used simultaneously by
both the host and guest.  If PT is in system mode, the host is the sole
owner of PT and the MSRs should never be passed through to the guest.

Luckily the missed check only results in unnecessary work, as select RTIT
MSRs are passed through only when RTIT tracing is enabled "in" the guest,
and tracing can't be enabled in the guest when KVM is in system mode
(writes to guest.MSR_IA32_RTIT_CTL are disallowed).

Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20220712015838.1253995-1-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/vmx/vmx.c

@@ -4004,7 +4004,9 @@ static void vmx_msr_filter_changed(struct kvm_vcpu *vcpu)
 			vmx_disable_intercept_for_msr(vcpu, msr, MSR_TYPE_W);
 	}
 
-	pt_update_intercept_for_msr(vcpu);
+	/* PT MSRs can be passed through iff PT is exposed to the guest. */
+	if (vmx_pt_mode_is_host_guest())
+		pt_update_intercept_for_msr(vcpu);
 }
 
 static inline void kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,