Re: [PATCH 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest

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

On 7/9/2025 1:37 PM, Kai Huang wrote:
> Reject KVM_SET_TSC_KHZ vCPU ioctl if guest's TSC is protected and not
> changeable by KVM.
> 
> For such TSC protected guests, e.g. TDX guests, typically the TSC is
> configured once at VM level before any vCPU are created and remains
> unchanged during VM's lifetime.  KVM provides the KVM_SET_TSC_KHZ VM
> scope ioctl to allow the userspace VMM to configure the TSC of such VM.
> After that the userspace VMM is not supposed to call the KVM_SET_TSC_KHZ
> vCPU scope ioctl anymore when creating the vCPU.
> 
> The de facto userspace VMM Qemu does this for TDX guests.  The upcoming
> SEV-SNP guests with Secure TSC should follow.
> 
> Note this could be a break of ABI.  But for now only TDX guests are TSC
> protected and only Qemu supports TDX, thus in practice this should not
> break any existing userspace.
> 
> 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 2806f7104295..699ca5e74bba 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6186,6 +6186,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>   		u32 user_tsc_khz;
>   
>   		r = -EINVAL;

I wondered if -EPERM is more appropriate. But I search the KVM files and 
find the KVM convention is to return -EINVAL.

> +		if (vcpu->arch.guest_tsc_protected)
> +			goto out;
> +
>   		user_tsc_khz = (u32)arg;
>   
>   		if (kvm_caps.has_tsc_control &&