[RFC PATCH v2 39/44] OvmfPkg: Move the GHCB allocations into reserved memory

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

From: Tom Lendacky <thomas.lendacky@amd.com>

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

After having transitioned from UEFI to the OS, the OS will need to boot
the APs. For an SEV-ES guest, the APs will have been parked by UEFI using
GHCB pages allocated by UEFI. The hypervisor will write to the GHCB
SW_EXITINFO2 field of the GHCB when the AP is booted. As a result, the
GHCB pages must be marked reserved so that the OS does not attempt to use
them and experience memory corruption because of the hypervisor write.

Change the GHCB allocation from the default boot services memory to
reserved memory.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/PlatformPei/AmdSev.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/OvmfPkg/PlatformPei/AmdSev.c b/OvmfPkg/PlatformPei/AmdSev.c
index d6733447bdf2..53eeb59702ae 100644
--- a/OvmfPkg/PlatformPei/AmdSev.c
+++ b/OvmfPkg/PlatformPei/AmdSev.c
@@ -49,9 +49,11 @@ AmdSevEsInitialize (
 
   //
   // Allocate GHCB pages.
+  //   Since the pages must survive across the UEFI to OS transition
+  //   make them reserved.
   //
   GhcbPageCount = mMaxCpuCount * 2;
-  GhcbBase = AllocatePages (GhcbPageCount);
+  GhcbBase = AllocateReservedPages (GhcbPageCount);
   ASSERT (GhcbBase);
 
   GhcbBasePa = (PHYSICAL_ADDRESS)(UINTN) GhcbBase;
-- 
2.17.1