OvmfPkg/MemEncryptSevLib: add support to validate system RAM

OvmfPkg/MemEncryptSevLib: add support to validate system RAM

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

Many of the integrity guarantees of SEV-SNP are enforced through the
Reverse Map Table (RMP). Each RMP entry contains the GPA at which a
particular page of DRAM should be mapped. The guest can request the
hypervisor to add pages in the RMP table via the Page State Change VMGEXIT
defined in the GHCB specification section 2.5.1 and 4.1.6. Inside each RMP
entry is a Validated flag; this flag is automatically cleared to 0 by the
CPU hardware when a new RMP entry is created for a guest. Each VM page
can be either validated or invalidated, as indicated by the Validated
flag in the RMP entry. Memory access to a private page that is not
validated generates a #VC. A VM can use the PVALIDATE instruction to
validate the private page before using it.

During the guest creation, the boot ROM memory is pre-validated by the
AMD-SEV firmware. The MemEncryptSevSnpValidateSystemRam() can be called
during the SEC and PEI phase to validate the detected system RAM.

One of the fields in the Page State Change NAE is the RMP page size. The
page size input parameter indicates that either a 4KB or 2MB page should
be used while adding the RMP entry. During the validation, when possible,
the MemEncryptSevSnpValidateSystemRam() will use the 2MB entry. A
hypervisor backing the memory may choose to use the different page size
in the RMP entry. In those cases, the PVALIDATE instruction should return
SIZEMISMATCH. If a SIZEMISMATCH is detected, then validate all 512-pages
constituting a 2MB region.

Upon completion, the PVALIDATE instruction sets the rFLAGS.CF to 0 if
instruction changed the RMP entry and to 1 if the instruction did not
change the RMP entry. The rFlags.CF will be 1 only when a memory region
is already validated. We should not double validate a memory
as it could lead to a security compromise. If double validation is
detected, terminate the boot.

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>