Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

Huang, Kai
2024-04-18T00:47:13Z

On 18/04/2024 11:35 am, Sean Christopherson wrote:
> On Thu, Apr 18, 2024, Kai Huang wrote:
>> On 18/04/2024 2:40 am, Sean Christopherson wrote:
>>> This way, architectures that aren't saddled with out-of-tree hypervisors can do
>>> the dead simple thing of enabling hardware during their initialization sequence,
>>> and the TDX code is much more sane, e.g. invoke kvm_x86_enable_virtualization()
>>> during late_hardware_setup(), and kvm_x86_disable_virtualization() during module
>>> exit (presumably).
>>
>> Fine to me, given I am not familiar with other ARCHs, assuming always enable
>> virtualization when KVM present is fine to them. :-)
>>
>> Two questions below:
>>
>>> +int kvm_x86_enable_virtualization(void)
>>> +{
>>> + int r;
>>> +
>>> + guard(mutex)(&vendor_module_lock);
>>
>> It's a little bit odd to take the vendor_module_lock mutex.
>>
>> It is called by kvm_arch_init_vm(), so more reasonablly we should still use
>> kvm_lock?
>
> I think this should take an x86-specific lock, since it's guarding x86-specific
> data.

OK. This makes sense.

And vendor_module_lock fits the bill perfectly. Well, except for the
> name, and I definitely have no objection to renaming it.

No opinion on renaming. Personally I wouldn't bother to rename. We can
add a comment in kvm_x86_enable_virtualization() to explain. Perhaps in
the future we just want to change to always enable virtualization for
x86 too..

>
>> Also, if we invoke kvm_x86_enable_virtualization() from
>> kvm_x86_ops->late_hardware_setup(), then IIUC we will deadlock here because
>> kvm_x86_vendor_init() already takes the vendor_module_lock?
>
> Ah, yeah. Oh, duh. I think the reason I didn't initially suggest late_hardware_setup()
> is that I was assuming/hoping TDX setup could be done after kvm_x86_vendor_exit().
> E.g. in vt_init() or whatever it gets called:
>
> r = kvm_x86_vendor_exit(...);
> if (r)
> return r;
>
> if (enable_tdx) {
> r = tdx_blah_blah_blah();
> if (r)
> goto vendor_exit;
> }

I assume the reason you introduced the late_hardware_setup() is purely
because you want to do:

cpu_emergency_register_virt_callback(kvm_x86_ops.emergency_enable);

after

kvm_ops_update()?

Anyway, we can also do 'enable_tdx' outside of kvm_x86_vendor_init() as
above, given it cannot be done in hardware_setup() anyway.

If we do 'enable_tdx' in late_hardware_setup(), we will need a
kvm_x86_enable_virtualization_nolock(), but that's also not a problem to me.

So which way do you prefer?

Btw, with kvm_x86_virtualization_enable(), it seems the compatibility
check is lost, which I assume is OK?

Btw2, currently tdx_enable() requires cpus_read_lock() must be called
prior. If we do unconditional tdx_cpu_enable() in vt_hardware_enable(),
then with your proposal IIUC there's no such requirement anymore,
because no task will be scheduled to the new CPU before it reaches
CPUHP_AP_ACTIVE. But now calling cpus_read_lock()/unlock() around
tdx_enable() also acceptable to me.

[...]

>>
>>> +int kvm_enable_virtualization(void)
>>> {
>>> + int r;
>>> +
>>> + r = cpuhp_setup_state(CPUHP_AP_KVM_ONLINE, "kvm/cpu:online",
>>> + kvm_online_cpu, kvm_offline_cpu);
>>> + if (r)
>>> + return r;
>>> +
>>> + register_syscore_ops(&kvm_syscore_ops);
>>> +
>>> + /*
>>> + * Manually undo virtualization enabling if the system is going down.
>>> + * If userspace initiated a forced reboot, e.g. reboot -f, then it's
>>> + * possible for an in-flight module load to enable virtualization
>>> + * after syscore_shutdown() is called, i.e. without kvm_shutdown()
>>> + * being invoked. Note, this relies on system_state being set _before_
>>> + * kvm_shutdown(), e.g. to ensure either kvm_shutdown() is invoked
>>> + * or this CPU observes the impedning shutdown. Which is why KVM uses
>>> + * a syscore ops hook instead of registering a dedicated reboot
>>> + * notifier (the latter runs before system_state is updated).
>>> + */
>>> + if (system_state == SYSTEM_HALT || system_state == SYSTEM_POWER_OFF ||
>>> + system_state == SYSTEM_RESTART) {
>>> + unregister_syscore_ops(&kvm_syscore_ops);
>>> + cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
>>> + return -EBUSY;
>>> + }
>>> +
>>
>> Aren't we also supposed to do:
>>
>> on_each_cpu(__kvm_enable_virtualization, NULL, 1);
>>
>> here?
>
> No, cpuhp_setup_state() invokes the callback, kvm_online_cpu(), on each CPU.
> I.e. KVM has been doing things the hard way by using cpuhp_setup_state_nocalls().
> That's part of the complexity I would like to get rid of.

Ah, right :-)

Btw, why couldn't we do the 'system_state' check at the very beginning
of this function?