[PATCH v4 28/40] OvmfPkg/PlatformPei: Move early GDT into ram when SEV-ES is enabled

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

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

The SEV support will clear the C-bit from non-RAM areas.  The early GDT
lives in a non-RAM area, so when an exception occurs (like a #VC) the GDT
will be read as un-encrypted even though it is encrypted. This will result
in a failure to be able to handle the exception.

Move the GDT into RAM so it can be accessed without error when running as
an SEV-ES guest.

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 | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/OvmfPkg/PlatformPei/AmdSev.c b/OvmfPkg/PlatformPei/AmdSev.c
index 900b0d977d61..b3fd2d86541a 100644
--- a/OvmfPkg/PlatformPei/AmdSev.c
+++ b/OvmfPkg/PlatformPei/AmdSev.c
@@ -37,6 +37,8 @@ AmdSevEsInitialize (
   PHYSICAL_ADDRESS  GhcbBasePa;
   UINTN             GhcbPageCount;
   RETURN_STATUS     PcdStatus, DecryptStatus;
+  IA32_DESCRIPTOR   Gdtr;
+  VOID              *Gdt;
 
   if (!MemEncryptSevEsIsEnabled ()) {
     return;
@@ -74,6 +76,22 @@ AmdSevEsInitialize (
     (UINT64)GhcbPageCount, GhcbBase));
 
   AsmWriteMsr64 (MSR_SEV_ES_GHCB, GhcbBasePa);
+
+  //
+  // The SEV support will clear the C-bit from non-RAM areas.  The early GDT
+  // lives in a non-RAM area, so when an exception occurs (like a #VC) the GDT
+  // will be read as un-encrypted even though it was created before the C-bit
+  // was cleared (encrypted). This will result in a failure to be able to
+  // handle the exception.
+  //
+  AsmReadGdtr (&Gdtr);
+
+  Gdt = AllocatePages (EFI_SIZE_TO_PAGES ((UINTN) Gdtr.Limit + 1));
+  ASSERT (Gdt != NULL);
+
+  CopyMem (Gdt, (VOID *) Gdtr.Base, Gdtr.Limit + 1);
+  Gdtr.Base = (UINTN) Gdt;
+  AsmWriteGdtr (&Gdtr);
 }
 
 /**
-- 
2.17.1