KVM: vmx: implement MSR_IA32_TSX_CTRL disable RTM functionality

The current guest mitigation of TAA is both too heavy and not really
sufficient.  It is too heavy because it will cause some affected CPUs
(those that have MDS_NO but lack TAA_NO) to fall back to VERW and
get the corresponding slowdown.  It is not really sufficient because
it will cause the MDS_NO bit to disappear upon microcode update, so
that VMs started before the microcode update will not be runnable
anymore afterwards, even with tsx=on.

Instead, if tsx=on on the host, we can emulate MSR_IA32_TSX_CTRL for
the guest and let it run without the VERW mitigation.  Even though
MSR_IA32_TSX_CTRL is quite heavyweight, and we do not want to write
it on every vmentry, we can use the shared MSR functionality because
the host kernel need not protect itself from TSX-based side-channels.
Tested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -450,6 +450,7 @@ const u32 vmx_msr_index[] = {
 	MSR_SYSCALL_MASK, MSR_LSTAR, MSR_CSTAR,
 #endif
 	MSR_EFER, MSR_TSC_AUX, MSR_STAR,
+	MSR_IA32_TSX_CTRL,
 };
 
 #if IS_ENABLED(CONFIG_HYPERV)
@@ -1683,6 +1684,9 @@ static void setup_msrs(struct vcpu_vmx *vmx)
 	index = __find_msr_index(vmx, MSR_TSC_AUX);
 	if (index >= 0 && guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDTSCP))
 		move_msr_up(vmx, index, save_nmsrs++);
+	index = __find_msr_index(vmx, MSR_IA32_TSX_CTRL);
+	if (index >= 0)
+		move_msr_up(vmx, index, save_nmsrs++);
 
 	vmx->save_nmsrs = save_nmsrs;
 	vmx->guest_msrs_ready = false;
@@ -1782,6 +1786,11 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 #endif
 	case MSR_EFER:
 		return kvm_get_msr_common(vcpu, msr_info);
+	case MSR_IA32_TSX_CTRL:
+		if (!msr_info->host_initiated &&
+		    !(vcpu->arch.arch_capabilities & ARCH_CAP_TSX_CTRL_MSR))
+			return 1;
+		goto find_shared_msr;
 	case MSR_IA32_UMWAIT_CONTROL:
 		if (!msr_info->host_initiated && !vmx_has_waitpkg(vmx))
 			return 1;
@@ -1884,8 +1893,9 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (!msr_info->host_initiated &&
 		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
 			return 1;
-		/* Else, falls through */
+		goto find_shared_msr;
 	default:
+	find_shared_msr:
 		msr = find_msr_entry(vmx, msr_info->index);
 		if (msr) {
 			msr_info->data = msr->data;
@@ -2001,6 +2011,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 					      MSR_IA32_SPEC_CTRL,
 					      MSR_TYPE_RW);
 		break;
+	case MSR_IA32_TSX_CTRL:
+		if (!msr_info->host_initiated &&
+		    !(vcpu->arch.arch_capabilities & ARCH_CAP_TSX_CTRL_MSR))
+			return 1;
+		if (data & ~(TSX_CTRL_RTM_DISABLE | TSX_CTRL_CPUID_CLEAR))
+			return 1;
+		goto find_shared_msr;
 	case MSR_IA32_PRED_CMD:
 		if (!msr_info->host_initiated &&
 		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
@@ -2152,8 +2169,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		/* Check reserved bit, higher 32 bits should be zero */
 		if ((data >> 32) != 0)
 			return 1;
-		/* Else, falls through */
+		goto find_shared_msr;
+
 	default:
+	find_shared_msr:
 		msr = find_msr_entry(vmx, msr_index);
 		if (msr) {
 			u64 old_msr_data = msr->data;
@@ -4234,7 +4253,20 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
 			continue;
 		vmx->guest_msrs[j].index = i;
 		vmx->guest_msrs[j].data = 0;
-		vmx->guest_msrs[j].mask = -1ull;
+
+		switch (index) {
+		case MSR_IA32_TSX_CTRL:
+			/*
+			 * No need to pass TSX_CTRL_CPUID_CLEAR through, so
+			 * let's avoid changing CPUID bits under the host
+			 * kernel's feet.
+			 */
+			vmx->guest_msrs[j].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
+			break;
+		default:
+			vmx->guest_msrs[j].mask = -1ull;
+			break;
+		}
 		++vmx->nmsrs;
 	}
 

arch/x86/kvm/x86.c

@@ -1314,29 +1314,16 @@ static u64 kvm_get_arch_capabilities(void)
 		data |= ARCH_CAP_MDS_NO;
 
 	/*
-	 * On TAA affected systems, export MDS_NO=0 when:
-	 *	- TSX is enabled on the host, i.e. X86_FEATURE_RTM=1.
-	 *	- Updated microcode is present. This is detected by
-	 *	  the presence of ARCH_CAP_TSX_CTRL_MSR and ensures
-	 *	  that VERW clears CPU buffers.
-	 *
-	 * When MDS_NO=0 is exported, guests deploy clear CPU buffer
-	 * mitigation and don't complain:
-	 *
-	 *	"Vulnerable: Clear CPU buffers attempted, no microcode"
-	 *
-	 * If TSX is disabled on the system, guests are also mitigated against
-	 * TAA and clear CPU buffer mitigation is not required for guests.
+	 * On TAA affected systems:
+	 *      - nothing to do if TSX is disabled on the host.
+	 *      - we emulate TSX_CTRL if present on the host.
+	 *	  This lets the guest use VERW to clear CPU buffers.
 	 */
 	if (!boot_cpu_has(X86_FEATURE_RTM))
-		data &= ~ARCH_CAP_TAA_NO;
+		data &= ~(ARCH_CAP_TAA_NO | ARCH_CAP_TSX_CTRL_MSR);
 	else if (!boot_cpu_has_bug(X86_BUG_TAA))
 		data |= ARCH_CAP_TAA_NO;
-	else if (data & ARCH_CAP_TSX_CTRL_MSR)
-		data &= ~ARCH_CAP_MDS_NO;
 
-	/* KVM does not emulate MSR_IA32_TSX_CTRL.  */
-	data &= ~ARCH_CAP_TSX_CTRL_MSR;
 	return data;
 }
 EXPORT_SYMBOL_GPL(kvm_get_arch_capabilities);