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 <seanjc@google.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>
Link: https://lore.kernel.org/r/135a35223ce8d01cea06b6cef30bfe494ec85827.1752444335.git.kai.huang@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 6be1ddedec492097fc941b27050928c82988139d..dcddef1dd47be4c356f01df8e984ec9e10a74881 100644 (file)
--- 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 912260e3725d54df42b12f868b2557b0a324db58..a247f67954eabf22e5869b6d9b585735b80ed9c6 100644 (file)
--- 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: {