[PATCH] UefiCpuPkg/MtrrLib: Add flag to skip disabling MTRRs prior to MTRR change.

[Leo Duran <leo.duran@amd.com>]

The default behavior is to disable MTRRs prior to an MTRR change.
However, on SMT platforms with shared CPU resources it may be desirable to
skip the default behavior, and leave the current state of the Enable bit.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leo Duran <leo.duran@amd.com>
---
 UefiCpuPkg/Library/MtrrLib/MtrrLib.c   | 10 +++++++---
 UefiCpuPkg/Library/MtrrLib/MtrrLib.inf |  3 +++
 UefiCpuPkg/UefiCpuPkg.dec              |  7 +++++++
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/UefiCpuPkg/Library/MtrrLib/MtrrLib.c b/UefiCpuPkg/Library/MtrrLib/MtrrLib.c
index dfce9a9..baf9a0f 100644
--- a/UefiCpuPkg/Library/MtrrLib/MtrrLib.c
+++ b/UefiCpuPkg/Library/MtrrLib/MtrrLib.c
@@ -6,6 +6,8 @@
     except for MtrrSetAllMtrrs() which is used to sync BSP's MTRR setting to APs.
 
   Copyright (c) 2008 - 2018, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2018, AMD Inc. All rights reserved.<BR>
+
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
   which accompanies this distribution.  The full text of the license may be found at
@@ -310,9 +312,11 @@ MtrrLibPreMtrrChange (
   //
   // Disable MTRRs
   //
-  DefType.Uint64 = AsmReadMsr64 (MSR_IA32_MTRR_DEF_TYPE);
-  DefType.Bits.E = 0;
-  AsmWriteMsr64 (MSR_IA32_MTRR_DEF_TYPE, DefType.Uint64);
+  if (!PcdGetBool (PcdSkipDisableMtrrsOnPreMtrrChange)) {
+    DefType.Uint64 = AsmReadMsr64 (MSR_IA32_MTRR_DEF_TYPE);
+    DefType.Bits.E = 0;
+    AsmWriteMsr64 (MSR_IA32_MTRR_DEF_TYPE, DefType.Uint64);
+  }
 }
 
 /**
diff --git a/UefiCpuPkg/Library/MtrrLib/MtrrLib.inf b/UefiCpuPkg/Library/MtrrLib/MtrrLib.inf
index a97cc61..06f33e8 100644
--- a/UefiCpuPkg/Library/MtrrLib/MtrrLib.inf
+++ b/UefiCpuPkg/Library/MtrrLib/MtrrLib.inf
@@ -2,6 +2,8 @@
 #  MTRR library provides APIs for MTRR operation.
 #
 #  Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
+#  Copyright (c) 2018, AMD Inc. All rights reserved.<BR>
+#
 #  This program and the accompanying materials
 #  are licensed and made available under the terms and conditions of the BSD License
 #  which accompanies this distribution.  The full text of the license may be found at
@@ -43,4 +45,5 @@
 
 [Pcd]
   gUefiCpuPkgTokenSpaceGuid.PcdCpuNumberOfReservedVariableMtrrs   ## SOMETIMES_CONSUMES
+  gUefiCpuPkgTokenSpaceGuid.PcdSkipDisableMtrrsOnPreMtrrChange    ## CONSUMES
 
diff --git a/UefiCpuPkg/UefiCpuPkg.dec b/UefiCpuPkg/UefiCpuPkg.dec
index 69d777a..64ec763 100644
--- a/UefiCpuPkg/UefiCpuPkg.dec
+++ b/UefiCpuPkg/UefiCpuPkg.dec
@@ -2,6 +2,7 @@
 # This Package provides UEFI compatible CPU modules and libraries.
 #
 # Copyright (c) 2007 - 2017, Intel Corporation. All rights reserved.<BR>
+# Copyright (c) 2018, AMD Inc. All rights reserved.<BR>
 #
 # This program and the accompanying materials are licensed and made available under
 # the terms and conditions of the BSD License which accompanies this distribution.
@@ -273,6 +274,12 @@
   # @Prompt Current boot is a power-on reset.
   gUefiCpuPkgTokenSpaceGuid.PcdIsPowerOnReset|FALSE|BOOLEAN|0x0000001B
 
+  ## Indicates desired behavior for disabling MTRRs prior to MTRR change.<BR><BR>
+  #   TRUE  - Skip disabling MTRRs prior to MTRR change.<BR>
+  #   FALSE - Disable MTRRs prior to MTRR change.<BR>
+  # @Prompt Desired behavior for disabling MTRRs prior to MTRR change.
+  gUefiCpuPkgTokenSpaceGuid.PcdSkipDisableMtrrsOnPreMtrrChange|FALSE|BOOLEAN|0x0000001E
+
 [PcdsDynamic, PcdsDynamicEx]
   ## Contains the pointer to a CPU S3 data buffer of structure ACPI_CPU_DATA.
   # @Prompt The pointer to a CPU S3 data buffer.
-- 
2.7.4