KVM: TDX: Require TDP MMU, mmio caching and EPT A/D bits for TDX

Disable TDX support when TDP MMU or mmio caching or EPT A/D bits aren't
supported.

As TDP MMU is becoming main stream than the legacy MMU, the legacy MMU
support for TDX isn't implemented.

TDX requires KVM mmio caching. Without mmio caching, KVM will go to MMIO
emulation without installing SPTEs for MMIOs. However, TDX guest is
protected and KVM would meet errors when trying to emulate MMIOs for TDX
guest during instruction decoding. So, TDX guest relies on SPTEs being
installed for MMIOs, which are with no RWX bits and with VE suppress bit
unset, to inject VE to TDX guest. The TDX guest would then issue TDVMCALL
in the VE handler to perform instruction decoding and have host do MMIO
emulation.

TDX also relies on EPT A/D bits as EPT A/D bits have been supported in all
CPUs since Haswell. Relying on it can avoid RWX bits being masked out in
the mirror page table for prefaulted entries.

Signed-off-by: Isaku Yamahata <[email protected]>
Co-developed-by: Rick Edgecombe <[email protected]>
Signed-off-by: Rick Edgecombe <[email protected]>
Co-developed-by: Yan Zhao <[email protected]>
Signed-off-by: Yan Zhao <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Yan Zhao <[email protected]>
---
Requested by Sean at [1].
[1] https://lore.kernel.org/kvm/[email protected]/
Signed-off-by: Paolo Bonzini <[email protected]>

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