From: Kai Huang Date: Sun, 13 Jul 2025 22:20:19 +0000 (+1200) Subject: KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=dcbe5a466c123a475bb66492749549f09b5cab00;p=thirdparty%2Flinux.git KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created Reject the KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created and update the documentation to reflect it. The VM scope KVM_SET_TSC_KHZ ioctl is used to set up the default TSC frequency that all subsequently created vCPUs can use. It is only intended to be called before any vCPU is created. Allowing it to be called after that only results in confusion but nothing good. Note this is an ABI change. But currently in Qemu (the de facto userspace VMM) only TDX uses this VM ioctl, and it is only called once before creating any vCPU, therefore the risk of breaking userspace is pretty low. Suggested-by: Sean Christopherson Signed-off-by: Kai Huang Reviewed-by: Xiaoyao Li Reviewed-by: Chao Gao Reviewed-by: Nikunj A Dadhania Link: https://lore.kernel.org/r/135a35223ce8d01cea06b6cef30bfe494ec85827.1752444335.git.kai.huang@intel.com Signed-off-by: Sean Christopherson --- diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 6be1ddedec492..dcddef1dd47be 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -2006,7 +2006,7 @@ frequency is KHz. If the KVM_CAP_VM_TSC_CONTROL capability is advertised, this can also be used as a vm ioctl to set the initial tsc frequency of subsequently -created vCPUs. +created vCPUs. Note, the vm ioctl is only allowed prior to creating vCPUs. 4.56 KVM_GET_TSC_KHZ -------------------- diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 912260e3725d5..a247f67954eab 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -7326,9 +7326,12 @@ set_pit2_out: if (user_tsc_khz == 0) user_tsc_khz = tsc_khz; - WRITE_ONCE(kvm->arch.default_tsc_khz, user_tsc_khz); - r = 0; - + mutex_lock(&kvm->lock); + if (!kvm->created_vcpus) { + WRITE_ONCE(kvm->arch.default_tsc_khz, user_tsc_khz); + r = 0; + } + mutex_unlock(&kvm->lock); goto out; } case KVM_GET_TSC_KHZ: {