KVM: VMX: Disable loading of TSX_CTRL MSR the more conventional way

Tag TSX_CTRL as not needing to be loaded when RTM isn't supported in the
host.  Crushing the write mask to '0' has the same effect, but requires
more mental gymnastics to understand.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210504171734.1434054-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -1771,7 +1771,13 @@ static void setup_msrs(struct vcpu_vmx *vmx)
 			   guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDTSCP) ||
 			   guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDPID));
 
-	vmx_setup_uret_msr(vmx, MSR_IA32_TSX_CTRL, true);
+	/*
+	 * hle=0, rtm=0, tsx_ctrl=1 can be found with some combinations of new
+	 * kernel and old userspace.  If those guests run on a tsx=off host, do
+	 * allow guests to use TSX_CTRL, but don't change the value in hardware
+	 * so that TSX remains always disabled.
+	 */
+	vmx_setup_uret_msr(vmx, MSR_IA32_TSX_CTRL, boot_cpu_has(X86_FEATURE_RTM));
 
 	if (cpu_has_vmx_msr_bitmap())
 		vmx_update_msr_bitmap(&vmx->vcpu);
@@ -6919,23 +6925,15 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
 		vmx->guest_uret_msrs[i].data = 0;
 		vmx->guest_uret_msrs[i].mask = -1ull;
 	}
-	tsx_ctrl = vmx_find_uret_msr(vmx, MSR_IA32_TSX_CTRL);
-	if (tsx_ctrl) {
+	if (boot_cpu_has(X86_FEATURE_RTM)) {
 		/*
 		 * TSX_CTRL_CPUID_CLEAR is handled in the CPUID interception.
 		 * Keep the host value unchanged to avoid changing CPUID bits
 		 * under the host kernel's feet.
-		 *
-		 * hle=0, rtm=0, tsx_ctrl=1 can be found with some combinations
-		 * of new kernel and old userspace.  If those guests run on a
-		 * tsx=off host, do allow guests to use TSX_CTRL, but do not
-		 * change the value on the host so that TSX remains always
-		 * disabled.
 		 */
-		if (boot_cpu_has(X86_FEATURE_RTM))
+		tsx_ctrl = vmx_find_uret_msr(vmx, MSR_IA32_TSX_CTRL);
+		if (tsx_ctrl)
 			vmx->guest_uret_msrs[i].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
-		else
-			vmx->guest_uret_msrs[i].mask = 0;
 	}
 
 	err = alloc_loaded_vmcs(&vmx->vmcs01);