[PATCH v4 06/30] KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

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@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
---
 arch/x86/kvm/svm/svm.c | 3 ++-
 arch/x86/kvm/vmx/vmx.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e7fdd7a9c280..7817752533fe 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5546,7 +5546,8 @@ 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 5c2c33a5f7dc..4f6035d72bbe 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8696,7 +8696,7 @@ __init int vmx_hardware_setup(void)
 	if (!enable_apicv || !cpu_has_vmx_ipiv())
 		enable_ipiv = false;
 
-	if (cpu_has_vmx_tsc_scaling())
+	if (cpu_has_vmx_tsc_scaling() && boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
 		kvm_caps.has_tsc_control = true;
 
 	kvm_caps.max_tsc_scaling_ratio = KVM_VMX_TSC_MULTIPLIER_MAX;
-- 
2.51.0