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/CcExitSvsm.h      | 29 ++++++++++++++++++++
 OvmfPkg/Library/CcExitLib/CcExitSvsm.c      | 21 ++++++++++++++
 OvmfPkg/Library/CcExitLib/CcExitVcHandler.c | 29 ++++++++++++++++++--
 3 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/OvmfPkg/Library/CcExitLib/CcExitSvsm.h b/OvmfPkg/Library/CcExitLib/CcExitSvsm.h
new file mode 100644
index 000000000000..2325e7a98910
--- /dev/null
+++ b/OvmfPkg/Library/CcExitLib/CcExitSvsm.h
@@ -0,0 +1,29 @@
+/** @file
+  Secure VM Service Module (SVSM) functions.
+
+  Copyright (C) 2024, Advanced Micro Devices, Inc. All rights reserved.<BR>
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+  @par Specification Reference:
+  Secure VM Service Module Specification
+
+**/
+
+#ifndef __CCEXITLIB_CCEXITSVSM_H__
+#define __CCEXITLIB_CCEXITSVSM_H__
+
+/**
+  Return the physical address of SVSM Call Area (CAA).
+
+  Determines the physical address of the SVSM CAA.
+
+  @return                         The physical address of the SVSM CAA
+
+**/
+UINT64
+EFIAPI
+SvsmGetCaaPa (
+  VOID
+  );
+
+#endif
diff --git a/OvmfPkg/Library/CcExitLib/CcExitSvsm.c b/OvmfPkg/Library/CcExitLib/CcExitSvsm.c
index 3459338b2033..e4c600d2a46b 100644
--- a/OvmfPkg/Library/CcExitLib/CcExitSvsm.c
+++ b/OvmfPkg/Library/CcExitLib/CcExitSvsm.c
@@ -44,6 +44,27 @@ SvsmTerminate (
   CpuDeadLoop ();
 }
 
+/**
+  Return the physical address of SVSM Call Area (CAA).
+
+  Determines the physical address of the SVSM CAA.
+
+  @return                         The physical address of the SVSM CAA
+
+**/
+UINT64
+EFIAPI
+SvsmGetCaaPa (
+  VOID
+  )
+{
+  SVSM_INFORMATION  *SvsmInfo;
+
+  SvsmInfo = (SVSM_INFORMATION *)(UINTN)PcdGet32 (PcdOvmfSnpSecretsBase);
+
+  return CcExitSnpSvsmPresent () ? SvsmInfo->SvsmCaa : 0;
+}
+
 /**
   Return the address of SVSM Call Area (CAA).
 
diff --git a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
index 0fc30f7bc4f6..950e7c34e37f 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
 
 **/
@@ -18,6 +18,7 @@
 
 #include "CcExitVcHandler.h"
 #include "CcInstruction.h"
+#include "CcExitSvsm.h"
 
 //
 // Non-automatic Exit function prototype
@@ -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 = SvsmGetCaaPa ();
+    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 (CcExitSnpSvsmPresent ()) {
+      *Eax |= BIT28;
+    }
   }
 
 Out:
-- 
2.42.0



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