Re: [PATCH 5/5] KVM: TDX: Add sub-ioctl KVM_TDX_TERMINATE_VM

[Adrian Hunter <adrian.hunter@intel.com>]

On 29/07/2025 22:33, Sean Christopherson wrote:
> +static int tdx_terminate_vm(struct kvm *kvm)
> +{
> +	if (kvm_trylock_all_vcpus(kvm))
> +		return -EBUSY;
> +
> +	kvm_vm_dead(kvm);
> +	to_kvm_tdx(kvm)->vm_terminated = true;
> +
> +	kvm_unlock_all_vcpus(kvm);
> +
> +	tdx_mmu_release_hkid(kvm);
> +
> +	return 0;
> +}

As I think I mentioned when removing vm_dead first came up,
I think we need more checks.  I spent some time going through
the code and came up with what is below:

First, we need to avoid TDX VCPU sub-IOCTLs from racing with
tdx_mmu_release_hkid().  But having any TDX sub-IOCTL run after
KVM_TDX_TERMINATE_VM raises questions of what might happen, so
it is much simpler to understand, if that is not possible.
There are 3 options:

1. Require that KVM_TDX_TERMINATE_VM is valid only if
kvm_tdx->state == TD_STATE_RUNNABLE.  Since currently all
the TDX sub-IOCTLs are for initialization, that would block
the opportunity for any to run after KVM_TDX_TERMINATE_VM.

2. Check vm_terminated in tdx_vm_ioctl() and tdx_vcpu_ioctl()

3. Test KVM_REQ_VM_DEAD in tdx_vm_ioctl() and tdx_vcpu_ioctl()

[ Note cannot check is_hkid_assigned() because that is racy ]

Secondly, I suggest we avoid SEAMCALLs that will fail and
result in KVM_BUG_ON() if HKID has been released.

There are 2 groups of those: MMU-related and TDVPS_ACCESSORS.

For the MMU-related, the following 2 functions should return
an error immediately if vm_terminated:

	tdx_sept_link_private_spt()
	tdx_sept_set_private_spte()

For that not be racy, extra synchronization is needed so that
vm_terminated can be reliably checked when holding mmu lock
i.e.

static int tdx_terminate_vm(struct kvm *kvm)
{
	if (kvm_trylock_all_vcpus(kvm))
		return -EBUSY;

	kvm_vm_dead(kvm);
+
+       write_lock(&kvm->mmu_lock);
	to_kvm_tdx(kvm)->vm_terminated = true;
+       write_unlock(&kvm->mmu_lock);

	kvm_unlock_all_vcpus(kvm);

	tdx_mmu_release_hkid(kvm);

	return 0;
}

Finally, there are 2 TDVPS_ACCESSORS that need avoiding:

	tdx_load_mmu_pgd()
		skip td_vmcs_write64() if vm_terminated

	tdx_protected_apic_has_interrupt()
		skip td_state_non_arch_read64() if vm_terminated