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

Kirill A. Shutemov
2023-12-05T10:54:42Z

On Mon, Dec 04, 2023 at 08:07:38PM +0100, Jeremi Piotrowski wrote:
> On 04/12/2023 10:17, Reshetova, Elena wrote:
> >> Check for additional CPUID bits to identify TDX guests running with Trust
> >> Domain (TD) partitioning enabled. TD partitioning is like nested virtualization
> >> inside the Trust Domain so there is a L1 TD VM(M) and there can be L2 TD VM(s).
> >>
> >> In this arrangement we are not guaranteed that the TDX_CPUID_LEAF_ID is
> >> visible
> >> to Linux running as an L2 TD VM. This is because a majority of TDX facilities
> >> are controlled by the L1 VMM and the L2 TDX guest needs to use TD partitioning
> >> aware mechanisms for what's left. So currently such guests do not have
> >> X86_FEATURE_TDX_GUEST set.
> >
> > Back to this concrete patch. Why cannot L1 VMM emulate the correct value of
> > the TDX_CPUID_LEAF_ID to L2 VM? It can do this per TDX partitioning arch.
> > How do you handle this and other CPUID calls call currently in L1? Per spec,
> > all CPUIDs calls from L2 will cause L2 --> L1 exit, so what do you do in L1?
> The disclaimer here is that I don't have access to the paravisor (L1) code. But
> to the best of my knowledge the L1 handles CPUID calls by calling into the TDX
> module, or synthesizing a response itself. TDX_CPUID_LEAF_ID is not provided to
> the L2 guest in order to discriminate a guest that is solely responsible for every
> TDX mechanism (running at L1) from one running at L2 that has to cooperate with L1.
> More below.
>
> >
> > Given that you do that simple emulation, you already end up with TDX guest
> > code being activated. Next you can check what features you wont be able to
> > provide in L1 and create simple emulation calls for the TDG calls that must be
> > supported and cannot return error. The biggest TDG call (TDVMCALL) is already
> > direct call into L0 VMM, so this part doesn’t require L1 VMM support.
>
> I don't see anything in the TD-partitioning spec that gives the TDX guest a way
> to detect if it's running at L2 or L1, or check whether TDVMCALLs go to L0/L1.
> So in any case this requires an extra cpuid call to establish the environment.
> Given that, exposing TDX_CPUID_LEAF_ID to the guest doesn't help.
>
> I'll give some examples of where the idea of emulating a TDX environment
> without attempting L1-L2 cooperation breaks down.
>
> hlt: if the guest issues a hlt TDVMCALL it goes to L0, but if it issues a classic hlt
> it traps to L1. The hlt should definitely go to L1 so that L1 has a chance to do
> housekeeping.

Why would L2 issue HLT TDVMCALL? It only happens in response to #VE, but
if partitioning enabled #VEs are routed to L1 anyway.

> map gpa: say the guest uses MAP_GPA TDVMCALL. This goes to L0, not L1 which is the actual
> entity that needs to have a say in performing the conversion. L1 can't act on the request
> if L0 would forward it because of the CoCo threat model. So L1 and L2 get out of sync.
> The only safe approach is for L2 to use a different mechanism to trap to L1 explicitly.

Hm? L1 is always in loop on share<->private conversion. I don't know why
you need MAP_GPA for that.

You can't rely on MAP_GPA anyway. It is optional (unfortunately). Conversion
doesn't require MAP_GPA call.

> Having a paravisor is required to support a TPM and having TDVMCALLs go to L0 is
> required to make performance viable for real workloads.
>
> >
> > Until we really see what breaks with this approach, I don’t think it is worth to
> > take in the complexity to support different L1 hypervisors view on partitioning.
> >
>
> I'm not asking to support different L1 hypervisors view on partitioning, I want to
> clean up the code (by fixing assumptions that no longer hold) for the model that I'm
> describing that: the kernel already supports, has an implementation that works and
> has actual users. This is also a model that Intel intentionally created the TD-partitioning
> spec to support.
>
> So lets work together to make X86_FEATURE_TDX_GUEST match reality.

I think the right direction is to make TDX architecture good enough
without that. If we need more hooks in TDX module that give required
control to L1, let's do that. (I don't see it so far)

--
Kiryl Shutsemau / Kirill A. Shutemov