diff mbox series

[RFC,v3,06/21] KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC

Message ID 20240522001817.619072-7-dwmw2@infradead.org (mailing list archive)
State New, archived
Headers show
Series Cleaning up the KVM clock mess | expand

Commit Message

David Woodhouse May 22, 2024, 12:17 a.m. UTC 
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 mbox series

Patch

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0f3b59da0d4a..4d3ec1c3231e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5202,7 +5202,8 @@  static __init int svm_hardware_setup(void)
 		kvm_enable_efer_bits(EFER_FFXSR);
 
 	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 6780313914f8..bee830adf744 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8428,7 +8428,7 @@  __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;