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RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3429 Intel TDX architecture does not prescribe a specific software convention to perform I/O from the guest TD. Guest TD providers have many choices to provide I/O to the guest. The common I/O models are emulated devices, para-virtualized devices, SRIOV devices and Direct Device assignments. TDVF chooses para-virtualized I/O (Choice-A) which use the TDG.VP.VMCALL function to invoke the funtions provided by the host VMM to perform I/O. Another choice (Choice-B) is the emulation performed by the #VE handler. There are 2 benefits of para-virtualized I/O: 1. Performance. VMEXIT/VMENTRY is skipped so that the performance is better than #VE handler. 2. De-couple with #VE handler. Choice-B depends on the #VE handler which means I/O is not available until #VE handler is installed. For example, in PEI phase #VE handler is installed in CpuMpPei, while communication with Qemu (via I/O port) happen earlier than it. IoLibInternalTdx.c provides the helper functions for Tdx guest. IoLibInternalTdxNull.c provides the null version of the helper functions. It is included in the Non-X64 IoLib so that the build will not be broken. Cc: Michael D Kinney <michael.d.kinney@intel.com> Cc: Liming Gao <gaoliming@byosoft.com.cn> Cc: Zhiguang Liu <zhiguang.liu@intel.com> Cc: Brijesh Singh <brijesh.singh@amd.com> Cc: Erdem Aktas <erdemaktas@google.com> Cc: James Bottomley <jejb@linux.ibm.com> Cc: Jiewen Yao <jiewen.yao@intel.com> Cc: Tom Lendacky <thomas.lendacky@amd.com> Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn> Reviewed-by: Jiewen Yao <jiewen.yao@intel.com> Signed-off-by: Min Xu <min.m.xu@intel.com>