[edk2-devel] [PATCH v2 22/23] Ovmfpkg/CcExitLib: Provide SVSM discovery support

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4654

The SVSM specification documents an alternative method of discovery for
the SVSM using a reserved CPUID bit and a reserved MSR.

For the CPUID support, the #VC handler of an SEV-SNP guest should modify
the returned value in the EAX register for the 0x8000001f CPUID function
by setting bit 28 when an SVSM is present.

For the MSR support, new reserved MSR 0xc001f000 has been defined. A #VC
should be generated when accessing this MSR. The #VC handler is expected
to ignore writes to this MSR and return the physical calling area address
(CAA) on reads of this MSR.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/Library/CcExitLib/CcExitLib.inf     |  3 +-
 OvmfPkg/Library/CcExitLib/SecCcExitLib.inf  |  3 +-
 OvmfPkg/Library/CcExitLib/CcExitVcHandler.c | 29 ++++++++++++++++++--
 3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/OvmfPkg/Library/CcExitLib/CcExitLib.inf b/OvmfPkg/Library/CcExitLib/CcExitLib.inf
index bc75cd5f5a04..692143acd9ad 100644
--- a/OvmfPkg/Library/CcExitLib/CcExitLib.inf
+++ b/OvmfPkg/Library/CcExitLib/CcExitLib.inf
@@ -1,7 +1,7 @@
 ## @file
 #  CcExitLib Library.
 #
-#  Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+#  Copyright (C) 2020 - 2024, Advanced Micro Devices, Inc. All rights reserved.<BR>
 #  Copyright (C) 2020 - 2022, Intel Corporation. All rights reserved.<BR>
 #  SPDX-License-Identifier: BSD-2-Clause-Patent
 #
@@ -41,6 +41,7 @@ [LibraryClasses]
   DebugLib
   LocalApicLib
   MemEncryptSevLib
+  CcSvsmLib
 
 [Pcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfCpuidBase
diff --git a/OvmfPkg/Library/CcExitLib/SecCcExitLib.inf b/OvmfPkg/Library/CcExitLib/SecCcExitLib.inf
index 811269dd2c06..6778c1af6516 100644
--- a/OvmfPkg/Library/CcExitLib/SecCcExitLib.inf
+++ b/OvmfPkg/Library/CcExitLib/SecCcExitLib.inf
@@ -1,7 +1,7 @@
 ## @file
 #  VMGEXIT Support Library.
 #
-#  Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+#  Copyright (C) 2020 - 2024, Advanced Micro Devices, Inc. All rights reserved.<BR>
 #  SPDX-License-Identifier: BSD-2-Clause-Patent
 #
 ##
@@ -41,6 +41,7 @@ [LibraryClasses]
   LocalApicLib
   MemEncryptSevLib
   PcdLib
+  CcSvsmLib
 
 [FixedPcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupBase
diff --git a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
index 0fc30f7bc4f6..edb4b57655d4 100644
--- a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
+++ b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
@@ -1,7 +1,7 @@
 /** @file
   X64 #VC Exception Handler functon.
 
-  Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+  Copyright (C) 2020 - 2024, Advanced Micro Devices, Inc. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -12,6 +12,7 @@
 #include <Library/LocalApicLib.h>
 #include <Library/MemEncryptSevLib.h>
 #include <Library/CcExitLib.h>
+#include <Library/CcSvsmLib.h>
 #include <Register/Amd/Msr.h>
 #include <Register/Intel/Cpuid.h>
 #include <IndustryStandard/InstructionParsing.h>
@@ -713,10 +714,29 @@ MsrExit (
   IN     CC_INSTRUCTION_DATA     *InstructionData
   )
 {
-  UINT64  ExitInfo1, Status;
+  MSR_SVSM_CAA_REGISTER  Msr;
+  UINT64                 ExitInfo1;
+  UINT64                 Status;
 
   ExitInfo1 = 0;
 
+  //
+  // The SVSM CAA MSR is a software implemented MSR and not supported
+  // by the hardware, handle it directly.
+  //
+  if (Regs->Rax == MSR_SVSM_CAA) {
+    // Writes to the SVSM CAA MSR are ignored
+    if (*(InstructionData->OpCodes + 1) == 0x30) {
+      return 0;
+    }
+
+    Msr.Uint64 = CcSvsmSnpGetCaa ();
+    Regs->Rax  = Msr.Bits.Lower32Bits;
+    Regs->Rdx  = Msr.Bits.Upper32Bits;
+
+    return 0;
+  }
+
   switch (*(InstructionData->OpCodes + 1)) {
     case 0x30: // WRMSR
       ExitInfo1          = 1;
@@ -1388,6 +1408,11 @@ GetCpuidFw (
     *Ebx = (*Ebx & 0xFFFFFF00) | (Ebx2 & 0x000000FF);
     /* node ID */
     *Ecx = (*Ecx & 0xFFFFFF00) | (Ecx2 & 0x000000FF);
+  } else if (EaxIn == 0x8000001F) {
+    /* Set the SVSM feature bit if running under an SVSM */
+    if (CcSvsmIsSvsmPresent ()) {
+      *Eax |= BIT28;
+    }
   }
 
 Out:
-- 
2.42.0



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