[RFC PATCH v3 38/43] OvmfPkg: Move the GHCB allocations into reserved memory

["Lendacky, Thomas" <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>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
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 b3fd2d86541a..84b337325d5c 100644
--- a/OvmfPkg/PlatformPei/AmdSev.c
+++ b/OvmfPkg/PlatformPei/AmdSev.c
@@ -49,9 +49,11 @@ AmdSevEsInitialize (
 
   //
   // Allocate GHCB and per-CPU variable 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 != NULL);
 
   GhcbBasePa = (PHYSICAL_ADDRESS)(UINTN) GhcbBase;
-- 
2.17.1