Re: [PATCH V4 1/1] KVM: TDX: Add sub-ioctl KVM_TDX_TERMINATE_VM

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

On 18/06/2025 09:00, Vishal Annapurve wrote:
> On Tue, Jun 17, 2025 at 10:50 PM Adrian Hunter <adrian.hunter@intel.com> wrote:
>> ...
>>>>
>>>> Changes in V4:
>>>>
>>>>         Drop TDX_FLUSHVP_NOT_DONE change.  It will be done separately.
>>>>         Use KVM_BUG_ON() instead of WARN_ON().
>>>>         Correct kvm_trylock_all_vcpus() return value.
>>>>
>>>> Changes in V3:
>>>>
>>>>         Remove KVM_BUG_ON() from tdx_mmu_release_hkid() because it would
>>>>         trigger on the error path from __tdx_td_init()
>>>>
>>>>         Put cpus_read_lock() handling back into tdx_mmu_release_hkid()
>>>>
>>>>         Handle KVM_TDX_TERMINATE_VM in the switch statement, i.e. let
>>>>         tdx_vm_ioctl() deal with kvm->lock
>>>> ....
>>>>
>>>> +static int tdx_terminate_vm(struct kvm *kvm)
>>>> +{
>>>> +       if (kvm_trylock_all_vcpus(kvm))
>>>> +               return -EBUSY;
>>>> +
>>>> +       kvm_vm_dead(kvm);
>>>
>>> With this no more VM ioctls can be issued on this instance. How would
>>> userspace VMM clean up the memslots? Is the expectation that
>>> guest_memfd and VM fds are closed to actually reclaim the memory?
>>
>> Yes
>>
>>>
>>> Ability to clean up memslots from userspace without closing
>>> VM/guest_memfd handles is useful to keep reusing the same guest_memfds
>>> for the next boot iteration of the VM in case of reboot.
>>
>> TD lifecycle does not include reboot.  In other words, reboot is
>> done by shutting down the TD and then starting again with a new TD.
>>
>> AFAIK it is not currently possible to shut down without closing
>> guest_memfds since the guest_memfd holds a reference (users_count)
>> to struct kvm, and destruction begins when users_count hits zero.
>>
> 
> gmem link support[1] allows associating existing guest_memfds with new
> VM instances.
> 
> Breakdown of the userspace VMM flow:
> 1) Create a new VM instance before closing guest_memfd files.
> 2) Link existing guest_memfd files with the new VM instance. -> This
> creates new set of files backed by the same inode but associated with
> the new VM instance.

So what about:

2.5) Call KVM_TDX_TERMINATE_VM IOCTL

Memory reclaimed after KVM_TDX_TERMINATE_VM will be done efficiently,
so avoid causing it to be reclaimed earlier.

> 3) Close the older guest memfd handles -> results in older VM instance cleanup.
> 
> [1] https://lore.kernel.org/lkml/cover.1747368092.git.afranji@google.com/#t