KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even if tsx=off

Userspace that does not know about KVM_GET_MSR_FEATURE_INDEX_LIST
will generally use the default value for MSR_IA32_ARCH_CAPABILITIES.
When this happens and the host has tsx=on, it is possible to end up with
virtual machines that have HLE and RTM disabled, but TSX_CTRL available.

If the fleet is then switched to tsx=off, kvm_get_arch_capabilities()
will clear the ARCH_CAP_TSX_CTRL_MSR bit and it will not be possible to
use the tsx=off hosts as migration destinations, even though the guests
do not have TSX enabled.

To allow this migration, allow guests to write to their TSX_CTRL MSR,
while keeping the host MSR unchanged for the entire life of the guests.
This ensures that TSX remains disabled and also saves MSR reads and
writes, and it's okay to do because with tsx=off we know that guests will
not have the HLE and RTM features in their CPUID.  (If userspace sets
bogus CPUID data, we do not expect HLE and RTM to work in guests anyway).

Cc: stable@vger.kernel.org
Fixes: cbbaa272 ("KVM: x86: fix presentation of TSX feature in ARCH_CAPABILITIES")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -6860,11 +6860,20 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
 		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.
+			 * 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.
 			 */
-			vmx->guest_uret_msrs[j].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
+			if (boot_cpu_has(X86_FEATURE_RTM))
+				vmx->guest_uret_msrs[j].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
+			else
+				vmx->guest_uret_msrs[j].mask = 0;
 			break;
 		default:
 			vmx->guest_uret_msrs[j].mask = -1ull;

arch/x86/kvm/x86.c

@@ -1394,16 +1394,24 @@ static u64 kvm_get_arch_capabilities(void)
 	if (!boot_cpu_has_bug(X86_BUG_MDS))
 		data |= ARCH_CAP_MDS_NO;
 
-	/*
-	 * 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 | ARCH_CAP_TSX_CTRL_MSR);
-	else if (!boot_cpu_has_bug(X86_BUG_TAA))
+	if (!boot_cpu_has(X86_FEATURE_RTM)) {
+		/*
+		 * If RTM=0 because the kernel has disabled TSX, the host might
+		 * have TAA_NO or TSX_CTRL.  Clear TAA_NO (the guest sees RTM=0
+		 * and therefore knows that there cannot be TAA) but keep
+		 * TSX_CTRL: some buggy userspaces leave it set on tsx=on hosts,
+		 * and we want to allow migrating those guests to tsx=off hosts.
+		 */
+		data &= ~ARCH_CAP_TAA_NO;
+	} else if (!boot_cpu_has_bug(X86_BUG_TAA)) {
 		data |= ARCH_CAP_TAA_NO;
+	} else {
+		/*
+		 * Nothing to do here; we emulate TSX_CTRL if present on the
+		 * host so the guest can choose between disabling TSX or
+		 * using VERW to clear CPU buffers.
+		 */
+	}
 
 	return data;
 }