https://edk2-docs.gitbook.io/edk-ii-build-specification/2_design_discussion/23_boot_sequence
> On Apr 20, 2021, at 11:34 PM, Eric van Tassell <evantass@amd.com> wrote:
> 
> 
> 
>> On 4/20/21 5:54 PM, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3345
>> The TPM support in OVMF performs MMIO accesses during the PEI phase. At
> 
> where are the phases defined and how many other are there?
> 
>> this point, MMIO ranges have not been marked un-encyrpted, so an SEV-ES
>> guest will fail attempting to perform MMIO to an encrypted address.
>> Read the PcdTpmBaseAddress and mark the specification defined range
>> (0x5000 in length) as un-encrypted, to allow an SEV-ES guest to process
>> the MMIO requests.
>> Cc: Laszlo Ersek <lersek@redhat.com>
>> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Brijesh Singh <brijesh.singh@amd.com>
>> Cc: James Bottomley <jejb@linux.ibm.com>
>> Cc: Jiewen Yao <jiewen.yao@intel.com>
>> Cc: Min Xu <min.m.xu@intel.com>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  OvmfPkg/PlatformPei/PlatformPei.inf |  1 +
>>  OvmfPkg/PlatformPei/AmdSev.c        | 19 +++++++++++++++++++
>>  2 files changed, 20 insertions(+)
>> diff --git a/OvmfPkg/PlatformPei/PlatformPei.inf b/OvmfPkg/PlatformPei/PlatformPei.inf
>> index 6ef77ba7bb21..de60332e9390 100644
>> --- a/OvmfPkg/PlatformPei/PlatformPei.inf
>> +++ b/OvmfPkg/PlatformPei/PlatformPei.inf
>> @@ -113,6 +113,7 @@ [Pcd]
>>    [FixedPcd]
>>    gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress
>> +  gEfiSecurityPkgTokenSpaceGuid.PcdTpmBaseAddress
>>    gEmbeddedTokenSpaceGuid.PcdMemoryTypeEfiACPIMemoryNVS
>>    gEmbeddedTokenSpaceGuid.PcdMemoryTypeEfiACPIReclaimMemory
>>    gEmbeddedTokenSpaceGuid.PcdMemoryTypeEfiReservedMemoryType
>> diff --git a/OvmfPkg/PlatformPei/AmdSev.c b/OvmfPkg/PlatformPei/AmdSev.c
>> index dddffdebda4b..d524929f9e10 100644
>> --- a/OvmfPkg/PlatformPei/AmdSev.c
>> +++ b/OvmfPkg/PlatformPei/AmdSev.c
>> @@ -141,6 +141,7 @@ AmdSevInitialize (
>>    )
>>  {
>>    UINT64                            EncryptionMask;
>> +  UINT64                            TpmBaseAddress;
>>    RETURN_STATUS                     PcdStatus;
>>      //
>> @@ -206,6 +207,24 @@ AmdSevInitialize (
>>      }
>>    }
>>  +  //
>> +  // PEI TPM support will perform MMIO accesses, be sure this range is not
>> +  // marked encrypted.
>> +  //
>> +  TpmBaseAddress = PcdGet64 (PcdTpmBaseAddress);
>> +  if (TpmBaseAddress != 0) {
>> +    RETURN_STATUS  DecryptStatus;
>> +
>> +    DecryptStatus = MemEncryptSevClearPageEncMask (
>> +                      0,
>> +                      TpmBaseAddress,
>> +                      EFI_SIZE_TO_PAGES (0x5000),
>> +                      FALSE
>> +                      );
>> +
>> +    ASSERT_RETURN_ERROR (DecryptStatus);
>> +  }
>> +
>>    //
>>    // Check and perform SEV-ES initialization if required.
>>    //
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