Re: [PATCH 2/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPU has been created

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/9/2025 1:38 PM, Kai Huang wrote:
> Reject the KVM_SET_TSC_KHZ VM ioctl when there's vCPU has already been
> created.
> 
> The VM scope KVM_SET_TSC_KHZ ioctl is used to set up the default TSC
> frequency that all subsequent created vCPUs 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>

> ---
>   arch/x86/kvm/x86.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 699ca5e74bba..e5e55d549468 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7194,6 +7194,10 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>   		u32 user_tsc_khz;
>   
>   		r = -EINVAL;
> +
> +		if (kvm->created_vcpus)
> +			goto out;
> +
>   		user_tsc_khz = (u32)arg;
>   
>   		if (kvm_caps.has_tsc_control &&