KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest

Reject KVM_SET_TSC_KHZ vCPU ioctl if guest's TSC is protected and not
changeable by KVM, and update the documentation to reflect it.

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, TDX support hasn't been fully released as of the "buggy" commit,
i.e. there is no established ABI to break.

Fixes: adafea1 ("KVM: x86: Add infrastructure for secure TSC")
Suggested-by: Sean Christopherson <[email protected]>
Signed-off-by: Kai Huang <[email protected]>
Reviewed-by: Xiaoyao Li <[email protected]>
Reviewed-by: Nikunj A Dadhania <[email protected]>
Link: https://lore.kernel.org/r/71bbdf87fdd423e3ba3a45b57642c119ee2dd98c.1752444335.git.kai.huang@intel.com
Signed-off-by: Sean Christopherson <[email protected]>

Author: kaihuang

Documentation/virt/kvm/api.rst

@@ -2008,6 +2008,13 @@ 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.
 
+For TSC protected Confidential Computing (CoCo) VMs where TSC frequency
+is configured once at VM scope and remains unchanged during VM's
+lifetime, the vm ioctl should be used to configure the TSC frequency
+and the vcpu ioctl is not supported.
+
+Example of such CoCo VMs: TDX guests.
+
 4.56 KVM_GET_TSC_KHZ
 --------------------
 

arch/x86/kvm/x86.c

@@ -6188,6 +6188,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		u32 user_tsc_khz;
 
 		r = -EINVAL;
+
+		if (vcpu->arch.guest_tsc_protected)
+			goto out;
+
 		user_tsc_khz = (u32)arg;
 
 		if (kvm_caps.has_tsc_control &&