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


Michael D Kinney
 

Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

-----Original Message-----
From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Ni, Ray
Sent: Tuesday, February 9, 2021 6:17 AM
To: devel@edk2.groups.io
Cc: Dong, Eric <eric.dong@intel.com>; Laszlo Ersek <lersek@redhat.com>; Kumar, Rahul1 <rahul1.kumar@intel.com>
Subject: [edk2-devel] [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, StackSizeLocation

diff --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



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