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

Under SEV-ES, a DR7 read or write intercept generates a #VC exception.
The #VC handler must provide special support to the guest for this. On
a DR7 write, the #VC handler must cache the value and issue a VMGEXIT
to notify the hypervisor of the write. However, the #VC handler must
not actually set the value of the DR7 register. On a DR7 read, the #VC
handler must return the cached value of the DR7 register to the guest.
VMGEXIT is not invoked for a DR7 register read.

To avoid exception recursion, a #VC exception will not try to read and
push the actual debug registers into the EFI_SYSTEM_CONTEXT_X64 struct
and instead push zeroes. The #VC exception handler does not make use of
the debug registers from saved context.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 .../X64/AMDSevVcCommon.c                      | 68 +++++++++++++++++++
 .../X64/ExceptionHandlerAsm.nasm              | 15 ++++
 2 files changed, 83 insertions(+)

diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/AMDSevVcCommon.c b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/AMDSevVcCommon.c
index 43a3a116af5d..8d7633b15e25 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/AMDSevVcCommon.c
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/AMDSevVcCommon.c
@@ -5,6 +5,12 @@
 
 #define CR4_OSXSAVE (1 << 18)
 
+#define DR7_RESET_VALUE 0x400
+typedef struct {
+  BOOLEAN  Dr7Cached;
+  UINT64   Dr7;
+} SEV_ES_PER_CPU_DATA;
+
 typedef enum {
   LongMode64Bit        = 0,
   LongModeCompat32Bit,
@@ -1043,6 +1049,60 @@ RdtscExit (
   return 0;
 }
 
+STATIC
+UINTN
+Dr7WriteExit (
+  GHCB                     *Ghcb,
+  EFI_SYSTEM_CONTEXT_X64   *Regs,
+  SEV_ES_INSTRUCTION_DATA  *InstructionData
+  )
+{
+  SEV_ES_INSTRUCTION_OPCODE_EXT  *Ext = &InstructionData->Ext;
+  SEV_ES_PER_CPU_DATA            *SevEsData = (SEV_ES_PER_CPU_DATA *) (Ghcb + 1);
+  INTN                           *Register;
+  UINTN                          Status;
+
+  DecodeModRm (Regs, InstructionData);
+
+  /* MOV DRn always treats MOD == 3 no matter how encoded */
+  Register = GetRegisterPointer (Regs, Ext->ModRm.Rm);
+
+  /* Using a value of 0 for ExitInfo1 means RAX holds the value */
+  Ghcb->SaveArea.Rax = *Register;
+  GhcbSetRegValid (Ghcb, GhcbRax);
+
+  Status = VmgExit (Ghcb, SvmExitDr7Write, 0, 0);
+  if (Status) {
+    return Status;
+  }
+
+  SevEsData->Dr7 = *Register;
+  SevEsData->Dr7Cached = TRUE;
+
+  return 0;
+}
+
+STATIC
+UINTN
+Dr7ReadExit (
+  GHCB                     *Ghcb,
+  EFI_SYSTEM_CONTEXT_X64   *Regs,
+  SEV_ES_INSTRUCTION_DATA  *InstructionData
+  )
+{
+  SEV_ES_INSTRUCTION_OPCODE_EXT  *Ext = &InstructionData->Ext;
+  SEV_ES_PER_CPU_DATA            *SevEsData = (SEV_ES_PER_CPU_DATA *) (Ghcb + 1);
+  INTN                           *Register;
+
+  DecodeModRm (Regs, InstructionData);
+
+  /* MOV DRn always treats MOD == 3 no matter how encoded */
+  Register = GetRegisterPointer (Regs, Ext->ModRm.Rm);
+  *Register = (SevEsData->Dr7Cached) ? SevEsData->Dr7 : DR7_RESET_VALUE;
+
+  return 0;
+}
+
 UINTN
 DoVcCommon (
   GHCB                *Ghcb,
@@ -1059,6 +1119,14 @@ DoVcCommon (
 
   ExitCode = Regs->ExceptionData;
   switch (ExitCode) {
+  case SvmExitDr7Read:
+    NaeExit = Dr7ReadExit;
+    break;
+
+  case SvmExitDr7Write:
+    NaeExit = Dr7WriteExit;
+    break;
+
   case SvmExitRdtsc:
     NaeExit = RdtscExit;
     break;
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
index 4db1a09f2881..d23af671df66 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
@@ -223,6 +223,9 @@ HasErrorCode:
     push    rax
 
 ;; UINT64  Dr0, Dr1, Dr2, Dr3, Dr6, Dr7;
+    cmp     qword [rbp + 8], 29
+    je      VcDebugRegs          ; For SEV-ES (#VC) Debug registers ignored
+
     mov     rax, dr7
     push    rax
     mov     rax, dr6
@@ -235,7 +238,19 @@ HasErrorCode:
     push    rax
     mov     rax, dr0
     push    rax
+    jmp     DrFinish
 
+VcDebugRegs:
+;; UINT64  Dr0, Dr1, Dr2, Dr3, Dr6, Dr7 are skipped for #VC to avoid exception recursion
+    xor     rax, rax
+    push    rax
+    push    rax
+    push    rax
+    push    rax
+    push    rax
+    push    rax
+
+DrFinish:
 ;; FX_SAVE_STATE_X64 FxSaveState;
     sub rsp, 512
     mov rdi, rsp
-- 
2.17.1


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