[PATCH v6 07/36] KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

KVM does make an attempt to cope with non-constant TSC, and has
notifiers to handle host TSC frequency changes. However, it *only*
adjusts the KVM clock, and doesn't adjust TSC frequency scaling when
the host changes.

This is presumably because non-constant TSCs were fixed in hardware
long before TSC scaling was implemented, so there should never be real
CPUs which have TSC scaling but *not* CONSTANT_TSC.

Such a combination could potentially happen in some odd L1 nesting
environment, but it isn't worth trying to support it. Just make the
dependency explicit.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Reviewed-by: Paul Durrant <paul@xxxxxxx>
---
 arch/x86/kvm/svm/svm.c |  3 ++-
 arch/x86/kvm/vmx/vmx.c | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e02a38da5296..c46a34aeb3df 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5557,7 +5557,8 @@ static __init int svm_hardware_setup(void)
 				     XFEATURE_MASK_BNDCSR);
 
 	if (tsc_scaling) {
-		if (!boot_cpu_has(X86_FEATURE_TSCRATEMSR)) {
+		if (!boot_cpu_has(X86_FEATURE_TSCRATEMSR) ||
+		    !boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
 			tsc_scaling = false;
 		} else {
 			pr_info("TSC scaling supported\n");
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b9103de01428..54e92d94155e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2792,6 +2792,16 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
 	if (!cpu_has_sgx())
 		_cpu_based_2nd_exec_control &= ~SECONDARY_EXEC_ENCLS_EXITING;
 
+	/*
+	 * KVM doesn't re-derive the TSC scaling ratio when the host TSC
+	 * frequency changes, so TSC scaling is only usable with a constant
+	 * TSC.  Clear the control here rather than in vmx_hardware_setup() so
+	 * that the per-CPU configs recomputed by vmx_check_processor_compat()
+	 * stay consistent with the golden vmcs_config.
+	 */
+	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
+		_cpu_based_2nd_exec_control &= ~SECONDARY_EXEC_TSC_SCALING;
+
 	if (_cpu_based_exec_control & CPU_BASED_ACTIVATE_TERTIARY_CONTROLS)
 		_cpu_based_3rd_exec_control =
 			adjust_vmx_controls64(KVM_OPTIONAL_VMX_TERTIARY_VM_EXEC_CONTROL,
-- 
2.54.0