UefiCpuPkg/PiSmmCpuDxeSmm: Improve the performance of GetFreeToken()

Today's GetFreeToken() runs at the algorithm complexity of O(n)
where n is the size of the token list.

The change introduces a new global variable FirstFreeToken and it
always points to the first free token. So the algorithm complexity
of GetFreeToken() decreases from O(n) to O(1).

The improvement matters when some SMI code uses StartupThisAP()
service for each of the AP such that the algorithm complexity
becomes O(n) * O(m) where m is the AP count.

As next steps,
1. PROCEDURE_TOKEN.Used field can be optimized out because
all tokens before FirstFreeToken should have "Used" set while all
after FirstFreeToken should have "Used" cleared.
2. ResetTokens() can be optimized to only reset tokens before
FirstFreeToken.

v2: add missing line in InitializeDataForMmMp.
v3: update copyright year to 2020.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
index fe7e8b0323..c9b3b739f3 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
@@ -1,7 +1,7 @@
 /** @file

 Agent Module to load other modules to deploy SMM Entry Vector for X86 CPU.

 

-Copyright (c) 2009 - 2019, Intel Corporation. All rights reserved.<BR>

+Copyright (c) 2009 - 2020, Intel Corporation. All rights reserved.<BR>

 Copyright (c) 2017, AMD Incorporated. All rights reserved.<BR>

 

 SPDX-License-Identifier: BSD-2-Clause-Patent

@@ -255,6 +255,7 @@ typedef struct {
 

   PROCEDURE_WRAPPER               *ApWrapperFunc;

   LIST_ENTRY                      TokenList;

+  LIST_ENTRY                      *FirstFreeToken;

 } SMM_CPU_PRIVATE_DATA;

 

 extern SMM_CPU_PRIVATE_DATA  *gSmmCpuPrivate;