Re: [PATCH v3 1/4] UefiCpuPkg/MpInitLib: Use XADD to avoid lock acquire/release

["Dong, Eric" <eric.dong@intel.com>]

Reviewed-by: Eric Dong <eric.dong@intel.com>

-----Original Message-----
From: Ni, Ray <ray.ni@intel.com> 
Sent: Tuesday, February 9, 2021 10:17 PM
To: devel@edk2.groups.io
Cc: Dong, Eric <eric.dong@intel.com>; Laszlo Ersek <lersek@redhat.com>; Kumar, Rahul1 <rahul1.kumar@intel.com>
Subject: [PATCH v3 1/4] UefiCpuPkg/MpInitLib: Use XADD to avoid lock acquire/release

When AP firstly wakes up, MpFuncs.nasm contains below logic to assign an unique ApIndex to each AP according to who comes first:
---ASM---
TestLock:
    xchg       [edi], eax
    cmp        eax, NotVacantFlag
    jz         TestLock

    mov        ecx, esi
    add        ecx, ApIndexLocation
    inc        dword [ecx]
    mov        ebx, [ecx]

Releaselock:
    mov        eax, VacantFlag
    xchg       [edi], eax
---ASM END---

"lock inc" cannot be used to increase ApIndex because not only the global ApIndex should be increased, but also the result should be stored to a local general purpose register EBX.

This patch learns from the NASM implementation of
InternalSyncIncrement() to use "XADD" instruction which can increase the global ApIndex and store the original ApIndex to EBX in one instruction.

With this patch, OVMF when running in a 255 threads QEMU spends about one second to wakeup all APs. Original implementation needs more than
10 seconds.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
---
 .../Library/MpInitLib/Ia32/MpFuncs.nasm       | 20 ++++++-------------
 UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm | 18 ++++++-----------
 2 files changed, 12 insertions(+), 26 deletions(-)

diff --git a/UefiCpuPkg/Library/MpInitLib/Ia32/MpFuncs.nasm b/UefiCpuPkg/Library/MpInitLib/Ia32/MpFuncs.nasm
index 7e81d24aa6..2eaddc93bc 100644
--- a/UefiCpuPkg/Library/MpInitLib/Ia32/MpFuncs.nasm
+++ b/UefiCpuPkg/Library/MpInitLib/Ia32/MpFuncs.nasm
@@ -1,5 +1,5 @@
 ;------------------------------------------------------------------------------ ;-; Copyright (c) 2015 - 2019, Intel Corporation. All rights reserved.<BR>+; Copyright (c) 2015 - 2021, Intel Corporation. All rights reserved.<BR> ; SPDX-License-Identifier: BSD-2-Clause-Patent ; ; Module Name:@@ -125,19 +125,11 @@ SkipEnableExecuteDisable:
     add        edi, LockLocation     mov        eax, NotVacantFlag -TestLock:-    xchg       [edi], eax-    cmp        eax, NotVacantFlag-    jz         TestLock--    mov        ecx, esi-    add        ecx, ApIndexLocation-    inc        dword [ecx]-    mov        ebx, [ecx]--Releaselock:-    mov        eax, VacantFlag-    xchg       [edi], eax+    mov        edi, esi+    add        edi, ApIndexLocation+    mov        ebx, 1+    lock xadd  dword [edi], ebx                 ; EBX = ApIndex+++    inc        ebx                              ; EBX is CpuNumber      mov        edi, esi     add        edi, StackSizeLocationdiff --git a/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm b/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm
index aecfd07bc0..5b588f2dcb 100644
--- a/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm
+++ b/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm
@@ -1,5 +1,5 @@
 ;------------------------------------------------------------------------------ ;-; Copyright (c) 2015 - 2019, Intel Corporation. All rights reserved.<BR>+; Copyright (c) 2015 - 2021, Intel Corporation. All rights reserved.<BR> ; SPDX-License-Identifier: BSD-2-Clause-Patent ; ; Module Name:@@ -161,18 +161,12 @@ LongModeStart:
     add        edi, LockLocation     mov        rax, NotVacantFlag -TestLock:-    xchg       qword [edi], rax-    cmp        rax, NotVacantFlag-    jz         TestLock--    lea        ecx, [esi + ApIndexLocation]-    inc        dword [ecx]-    mov        ebx, [ecx]+    mov        edi, esi+    add        edi, ApIndexLocation+    mov        ebx, 1+    lock xadd  dword [edi], ebx                 ; EBX = ApIndex+++    inc        ebx                              ; EBX is CpuNumber -Releaselock:-    mov        rax, VacantFlag-    xchg       qword [edi], rax     ; program stack     mov        edi, esi     add        edi, StackSizeLocation-- 
2.27.0.windows.1