Re: [PATCH v1 1/3] x86/tdx: Check for TDX partitioning during early TDX init

Jeremi Piotrowski
2023-12-07T19:35:58Z

On 07/12/2023 18:21, Jeremi Piotrowski wrote:
> On 07/12/2023 13:58, Huang, Kai wrote:
>>>
>>> That's how it currently works - all the enlightenments are in hypervisor/paravisor
>>> specific code in arch/x86/hyperv and drivers/hv and the vm is not marked with
>>> X86_FEATURE_TDX_GUEST.
>>
>> And I believe there's a reason that the VM is not marked as TDX guest.
> Yes, as Elena said:
> """
> OK, so in your case it is a decision of L1 VMM not to set the TDX_CPUID_LEAF_ID
> to reflect that it is a tdx guest and it is on purpose because you want to
> drop into a special tdx guest, i.e. partitioned guest.
> """
> TDX does not provide a means to let the partitioned guest know that it needs to
> cooperate with the paravisor (e.g. because TDVMCALLs are routed to L0) so this is
> exposed in a paravisor specific way (cpuids in patch 1).
>
>>
>>>
>>> But without X86_FEATURE_TDX_GUEST userspace has no unified way to discover that an
>>> environment is protected by TDX and also the VM gets classified as "AMD SEV" in dmesg.
>>> This is due to CC_ATTR_GUEST_MEM_ENCRYPT being set but X86_FEATURE_TDX_GUEST not.
>>
>> Can you provide more information about what does _userspace_ do here?
>
> I gave one usecase in a different email. A workload scheduler like Kubernetes might want to
> place a workload in a confidential environment, and needs a way to determine that a VM is
> TDX protected (or SNP protected) to make that placement decision.
>
>>
>> What's the difference if it sees a TDX guest or a normal non-coco guest in
>> /proc/cpuinfo?
>>
>> Looks the whole purpose of this series is to make userspace happy by advertising
>> TDX guest to /proc/cpuinfo. But if we do that we will have bad side-effect in
>> the kernel so that we need to do things in your patch 2/3.
>>
>
> Yes, exactly. It's unifying the two approaches so that userspace doesn't have to
> care.
>
>> That doesn't seem very convincing.
>
> Why not?
> The whole point of the kernel is to provide a unified interface to userspace and
> abstract away these small differences. Yes it requires some kernel code to do,
> thats not a reason to force every userspace to implement its own logic. This is
> what the flags in /proc/cpuinfo are for.
>

So I feel like we're finally getting to the gist of the disagreements in this thread.

Here's something I think we should all agree on (both a) and b)). X86_FEATURE_TDX_GUEST:
a) is visible to userspace and not just some kernel-only construct
b) means "this is a guest running in an Intel TDX Trust Domain, and said guest is aware
of TDX"

a) is obvious but I think needs restating. b) is what userspace expects, and excludes legacy
(/unmodified) guests running in a TD. That's a reasonable definition.

For kernel only checks we can rely on platform-specific CC_ATTRS checked through
intel_cc_platform_has.

@Borislav: does that sound reasonable to you?
@Kai, @Kirill, @Elena: can I get you to agree with this compromise, for userspace' sake?

Jeremi