2024-04-18 23:42:04

by David Woodhouse

Subject: [PATCH 05/10] KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC

From: David Woodhouse <[email protected]>

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 <[email protected]>
---
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 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;
--
2.44.0

2024-04-19 15:46:05

by Paul Durrant

Subject: Re: [PATCH 05/10] KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC

On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <[email protected]>
>
> 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 <[email protected]>
> ---
> arch/x86/kvm/svm/svm.c | 3 ++-
> arch/x86/kvm/vmx/vmx.c | 2 +-
> 2 files changed, 3 insertions(+), 2 deletions(-)
>

Reviewed-by: Paul Durrant <[email protected]>