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