OvmfPkg: reserve SNP secrets page

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

During the SNP guest launch sequence, a special secrets page needs to be
inserted by the VMM. The PSP will populate the page; it will contain the
VM Platform Communication Key (VMPCKs) used by the guest to send and
receive secure messages to the PSP.

The purpose of the secrets page in the SEV-SNP is different from the one
used in SEV guests. In SEV, the secrets page contains the guest owner's
private data after the remote attestation.

Cc: Michael Roth <michael.roth@amd.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>

OvmfPkg/ResetVector/X64/OvmfSevMetadata.asm

@@ -14,6 +14,9 @@ BITS  64
 ; The section must be accepted or validated by the VMM before the boot
 %define OVMF_SECTION_TYPE_SNP_SEC_MEM     0x1
 
+; AMD SEV-SNP specific sections
+%define OVMF_SECTION_TYPE_SNP_SECRETS     0x2
+
 ALIGN 16
 
 TIMES (15 - ((OvmfSevGuidedStructureEnd - OvmfSevGuidedStructureStart + 15) % 16)) DB 0
@@ -30,5 +33,11 @@ _DescriptorSev:
   DD OVMF_SEV_METADATA_VERSION                              ; Version
   DD (OvmfSevGuidedStructureEnd - _DescriptorSev - 16) / 12 ; Number of sections
 
+; SEV-SNP Secrets page
+SevSnpSecrets:
+  DD  SEV_SNP_SECRETS_BASE
+  DD  SEV_SNP_SECRETS_SIZE
+  DD  OVMF_SECTION_TYPE_SNP_SECRETS
+
 OvmfSevGuidedStructureEnd:
   ALIGN   16