Re: [PATCH 1/1] UefiCpuPkg: Save PcdConfidentialComputingGuestAttr in mCcGuestAttr

["Min Xu" <min.m.xu@intel.com>]

On April 29, 2022 11:07 AM, Ni Ray wrote:
> 
> Min,
> You cannot use C global variable in PEIM.
> Can you add a new field in _CPU_MP_DATA?
> 
The reason why C global variable cannot be used in PEIM is that in some scenario PEIM is executed in FLASH so that the value of C global variable cannot be kept in different calls. But I don't think it is a problem in this situation.
1. This global variable is to keep the PcdConfidentialComputingGuestAttribute in mCcGuestAttr. So if this is Tdx guest, then the global variable can be kept (CC_GUEST_IS_TDX (mCcGuestAttr) == TRUE). 
2. If this is Non-Td guest, then even the global variable cannot be kept, CC_GUEST_IS_TDX (mCcGuestAttr) is FALSE. So mCcGuestAttr can still work.

There is another solution that we can use CcProbe (which is in MdePkg/CcProbeLib). CcProbe checks the work area to fetch the guest type. It calls FixedPcdGet32 (PcdOvmfWorkAreaBase) so there is no SMP safe issue in PcdLib.

As to a new field in CPU_MP_DATA, Tdx guest doesn't create CPU_MP_DATA object in MpInitLibInitialize, instead it return right after it detects the working guest is Tdx guest. So this fix will be more complicated than above 2 solutions.

Ray, What's your thought?

Thanks
Min