OvmfPkg: Add README for TDVF

RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3249

Add README for TDVF.

Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

OvmfPkg/IntelTdx/README

@@ -0,0 +1,88 @@
+TDVF Overview
+-------------
+
+<b>Intel Trust Domain Extension (TDX)</b> is Intel Architecture extension
+to provide trusted, isolated VM execution by removing CSP software
+(hypervisor etc) from the TCB. <b>TDX Virtual Firmware (TDVF)</b> is an
+EDK II based project to enable UEFI support for TDX based Virtual
+Machines. It provides the capability to launch a TD.
+
+The <b>Intel? TDX Virtual Firmware Design Guide</b> is at
+https://www.intel.com/content/dam/develop/external/us/en/documents/tdx-virtual-firmware-design-guide-rev-1.01.pdf.
+
+More information can be found at:
+https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html
+
+
+Configurations and Features
+----------------------------
+
+There are 2 configurations for TDVF.
+
+<b>Config-A:</b>
+ - Merge the *basic* TDVF feature to existing OvmfX64Pkg.dsc. (Align
+   with existing SEV)
+ - Threat model: VMM is NOT out of TCB. (We don?t make things worse)
+ - The OvmfX64Pkg.dsc includes SEV/TDX/normal OVMF basic boot capability.
+   The final binary can run on SEV/TDX/normal OVMF.
+ - No changes to existing OvmfPkgX64 image layout.
+ - No need to add additional security features if they do not exist today.
+ - No need to remove features if they exist today.
+ - RTMR is not supported.
+ - PEI phase is NOT skipped in either Td or Non-Td.
+
+<b>Config-B:</b>
+ - (*) Add a standalone IntelTdx.dsc to a TDX specific directory for a *full*
+   feature TDVF.(Align with existing SEV)
+ - (*) Threat model: VMM is out of TCB. (We need necessary change to prevent
+   attack from VMM)
+ - (*) IntelTdx.dsc includes TDX/normal OVMF basic boot capability. The final
+   binary can run on TDX/normal OVMF.
+ - It might eventually merge with AmdSev.dsc, but NOT at this point of
+   time. And we don?t know when it will happen. We need sync with AMD in
+   the community after both of us think the solutions are mature to merge.
+ - Need to add necessary security feature as mandatory requirement, such
+   as RTMR based Trusted Boot support.
+ - Need to measure the external input from Host VMM, such as TdHob, CFV.
+ - Need to measure other external input, such as FW_CFG data, os loader,
+   initrd, etc.
+ - Need to remove unnecessary attack surfaces, such as network stack.
+
+In current stage, <b>Config-A</b> has been merged into edk2-master branch.
+The corresponding pkg file is OvmfPkg/OvmfPkgX64.dsc.
+
+<b>Config-B</b> is split into several waves. The corresponding pkg file is
+OvmfPkg/IntelTdx/IntelTdxX64.dsc. The features with (*) have been implemented
+and merged into edk2-master branch. Others are in upstreaming progress.
+
+Build
+------
+- Build the TDVF (Config-A) target:
+`cd /path/to/edk2`
+`source edksetup.sh`
+`build.sh -p OvmfPkg/OvmfPkgX64.dsc -a X64 -t GCC5`
+
+- Build the TDVF (Config-B) target:
+`cd /path/to/edk2`
+`set PACKAGES_PATH=/path/to/edk2/OvmfPkg`
+`source edksetup.sh`
+`build.sh -p OvmfPkg/IntelTdx/IntelTdxX64.dsc -a X64 -t GCC5`
+
+Usage
+-----
+
+Assuming TDX-QEMU/TDX-KVM are already built, one can start a TD virtual
+machine as [launching-td-guest](https://github.com/intel/qemu-tdx/blob/tdx-qemu-upstream-rfc-v3/docs/system/i386/tdx.rst#launching-a-td-tdx-vm):
+
+`qemu_system_x86 \`
+`   -machine ...,confidential-guest-support=tdx0 \`
+`   -object tdx-guest,id=tdx0,[sept-ve-disable=off] \`
+`   -drive if=pflash,format=raw,unit=0,file=/path/to/OVMF_CODE.fd \`
+`   -drive if=pflash,format=raw,unit=1,file=/path/to/OVMF_VARS.fd \`
+
+Note:
+TDX-QEMU/TDX-KVM are still in upstreaming progress. Please refer to:
+ - kvm  : https://github.com/intel/tdx/tree/kvm-upstream
+ - qemu : https://github.com/intel/qemu-tdx/blob/tdx-qemu-upstream-rfc-v3
+
+Once above 2 upstreaming are completed a minimum qemu/kvm version will be updated here.