OvmfPkg: Reserve a page in memory for the SEV-ES usage

OvmfPkg: Reserve a page in memory for the SEV-ES usage

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Reserve a fixed area of memory for SEV-ES use and set a fixed PCD,
PcdSevEsWorkAreaBase, to this value.

This area will be used by SEV-ES support for two purposes:
  1. Communicating the SEV-ES status during BSP boot to SEC:
     Using a byte of memory from the page, the BSP reset vector code can
     communicate the SEV-ES status to SEC for use before exception
     handling can be enabled in SEC. After SEC, this field is no longer
     valid and the standard way of determine if SEV-ES is active should
     be used.

  2. Establishing an area of memory for AP boot support:
     A hypervisor is not allowed to update an SEV-ES guest's register
     state, so when booting an SEV-ES guest AP, the hypervisor is not
     allowed to set the RIP to the guest requested value. Instead an
     SEV-ES AP must be re-directed from within the guest to the actual
     requested staring location as specified in the INIT-SIPI-SIPI
     sequence.

     Use this memory for reset vector code that can be programmed to have
     the AP jump to the desired RIP location after starting the AP. This
     is required for only the very first AP reset.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>