Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

[Yan Zhao]

On Sat, Jun 14, 2025 at 07:33:48AM +0800, Edgecombe, Rick P wrote:
> On Fri, 2025-06-13 at 15:19 -0700, Sean Christopherson wrote:
> > > Arg, I just realized a one-way opt-in will have a theoretical gap. If the
> > > guest
> > > kexec's, the new kernel will need to match the opt-in.
> > 
> > All the more reason to make this a property of the VM that is passed via
> > "struct td_params".  I.e. put the onus on the owner of the VM to ensure their
> > kernel(s) have been updated accordingly.
> 
> Hmm, it gives me pause. At minimum it should have an enumeration to the guest.
> 
> > 
> > I understand that this could be painful, but honestly _all_ of TDX and SNP is
> > painful for the guest.  E.g. I don't think it's any worse than the security
> > issues with TDX (and SNP) guests using kvmclock (which I'd love some reviews
> > on,
> > btw).
> > 
> > https://lore.kernel.org/all/20250227021855.3257188-35-seanjc@xxxxxxxxxx
> 
> Oh, nice. I hadn't seen this. Agree that a comprehensive guest setup is quite
> manual. But here we are playing with guest ABI. In practice, yes it's similar to
> passing yet another arg to get a good TD.
Could we introduce a TD attr TDX_ATTR_SEPT_EXPLICIT_DEMOTION?

It can be something similar to TDX_ATTR_SEPT_VE_DISABLE except that we don't
provide a dynamical way as the TDCS_CONFIG_FLEXIBLE_PENDING_VE to allow guest to
turn on/off SEPT_VE_DISABLE.
(See the disable_sept_ve() in ./arch/x86/coco/tdx/tdx.c).

So, if userspace configures a TD with TDX_ATTR_SEPT_EXPLICIT_DEMOTION, KVM first
checks if SEPT_EXPLICIT_DEMOTION is supported.
The guest can also check if it would like to support SEPT_EXPLICIT_DEMOTION to
determine to continue or shut down. (If it does not check SEPT_EXPLICIT_DEMOTION,
e.g., if we don't want to update EDK2, the guest must accept memory before
memory accessing).

- if TD is configured with SEPT_EXPLICIT_DEMOTION, KVM allows to map at 2MB when
  there's no level info in an EPT violation. The guest must accept memory before
  accessing memory or if it wants to accept only a partial of host's mapping, it
  needs to explicitly invoke a TDVMCALL to request KVM to perform page demotion.

- if TD is configured without SEPT_EXPLICIT_DEMOTION, KVM always maps at 4KB
  when there's no level info in an EPT violation.

- No matter SEPT_EXPLICIT_DEMOTION is configured or not, if there's a level info
  in an EPT violation, while KVM honors the level info as the max_level info,
  KVM ignores the demotion request in the fault path.

> We can start with a prototype the host side arg and see how it turns out. I
> realized we need to verify edk2 as well.
Current EDK2 should always accept pages before actual memory access.
So, I think it should be fine.