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>
---
 Documentation/virt/kvm/api.rst | 2 +-
 arch/x86/kvm/x86.c             | 9 ++++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 43ed57e048a8..e343430ccb01 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. The vm ioctl must be called before any vCPU is created.
 
 4.56 KVM_GET_TSC_KHZ
 --------------------
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2806f7104295..4051c0cacb92 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7199,9 +7199,12 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		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: {
-- 
2.50.0

On Mon, Jul 14, 2025 at 10:20:19AM +1200, Kai Huang wrote:
>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>

On 7/14/2025 3:50 AM, Kai Huang wrote:
> 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>

LTGM:

Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>

> ---
>  Documentation/virt/kvm/api.rst | 2 +-
>  arch/x86/kvm/x86.c             | 9 ++++++---
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 43ed57e048a8..e343430ccb01 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. The vm ioctl must be called before any vCPU is created.
>  
>  4.56 KVM_GET_TSC_KHZ
>  --------------------
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2806f7104295..4051c0cacb92 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7199,9 +7199,12 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>  		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: {