[PATCH v7 25/43] UefiCpuPkg/CpuExceptionHandler: Add support for DR7 Read/Write NAE events

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

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

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.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 .../X64/ArchAMDSevVcHandler.c                 | 105 ++++++++++++++++++
 .../X64/ExceptionHandlerAsm.nasm              |  17 +++
 2 files changed, 122 insertions(+)

diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ArchAMDSevVcHandler.c b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ArchAMDSevVcHandler.c
index 023e7dc31202..af5567d85593 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ArchAMDSevVcHandler.c
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ArchAMDSevVcHandler.c
@@ -13,6 +13,16 @@
 
 #define CR4_OSXSAVE (1 << 18)
 
+#define DR7_RESET_VALUE 0x400
+
+//
+// Per-CPU data mapping structure
+//
+typedef struct {
+  BOOLEAN  Dr7Cached;
+  UINT64   Dr7;
+} SEV_ES_PER_CPU_DATA;
+
 //
 // Instruction execution mode definition
 //
@@ -1487,6 +1497,93 @@ RdtscExit (
   return 0;
 }
 
+/**
+  Handle a DR7 register write event.
+
+  Use the VMGEXIT instruction to handle a DR7 write event.
+
+  @param[in, out] Ghcb             Pointer to the Guest-Hypervisor Communication
+                                   Block
+  @param[in, out] Regs             x64 processor context
+  @param[in]      InstructionData  Instruction parsing context
+
+  @retval 0                        Event handled successfully
+  @retval Others                   New exception value to propagate
+
+**/
+STATIC
+UINT64
+Dr7WriteExit (
+  IN OUT GHCB                     *Ghcb,
+  IN OUT EFI_SYSTEM_CONTEXT_X64   *Regs,
+  IN     SEV_ES_INSTRUCTION_DATA  *InstructionData
+  )
+{
+  SEV_ES_INSTRUCTION_OPCODE_EXT  *Ext;
+  SEV_ES_PER_CPU_DATA            *SevEsData;
+  INTN                           *Register;
+  UINT64                         Status;
+
+  Ext = &InstructionData->Ext;
+  SevEsData = (SEV_ES_PER_CPU_DATA *) (Ghcb + 1);
+
+  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;
+}
+
+/**
+  Handle a DR7 register read event.
+
+  Use the VMGEXIT instruction to handle a DR7 read event.
+
+  @param[in, out] Ghcb             Pointer to the Guest-Hypervisor Communication
+                                   Block
+  @param[in, out] Regs             x64 processor context
+  @param[in]      InstructionData  Instruction parsing context
+
+  @retval 0                        Event handled successfully
+
+**/
+STATIC
+UINT64
+Dr7ReadExit (
+  IN OUT GHCB                     *Ghcb,
+  IN OUT EFI_SYSTEM_CONTEXT_X64   *Regs,
+  IN     SEV_ES_INSTRUCTION_DATA  *InstructionData
+  )
+{
+  SEV_ES_INSTRUCTION_OPCODE_EXT  *Ext;
+  SEV_ES_PER_CPU_DATA            *SevEsData;
+  INTN                           *Register;
+
+  Ext = &InstructionData->Ext;
+  SevEsData = (SEV_ES_PER_CPU_DATA *) (Ghcb + 1);
+
+  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;
+}
+
 /**
   Common #VC exception handling routine.
 
@@ -1517,6 +1614,14 @@ 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 19198f273137..26cae56cc5cf 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
@@ -18,6 +18,8 @@
 ; CommonExceptionHandler()
 ;
 
+%define VC_EXCEPTION 29
+
 extern ASM_PFX(mErrorCodeFlag)    ; Error code flags for exceptions
 extern ASM_PFX(mDoFarReturnFlag)  ; Do far return flag
 extern ASM_PFX(CommonExceptionHandler)
@@ -225,6 +227,9 @@ HasErrorCode:
     push    rax
 
 ;; UINT64  Dr0, Dr1, Dr2, Dr3, Dr6, Dr7;
+    cmp     qword [rbp + 8], VC_EXCEPTION
+    je      VcDebugRegs          ; For SEV-ES (#VC) Debug registers ignored
+
     mov     rax, dr7
     push    rax
     mov     rax, dr6
@@ -237,7 +242,19 @@ 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