linux-kernel - BUG ? exc_page_fault() was optimized out of fred_hwexc() by gcc with default kernel build option (-O2).

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Message-ID:
 <TYZPR03MB88013AABBBC2B40F7B955825D1E52@TYZPR03MB8801.apcprd03.prod.outlook.com>
Date: Sat, 18 Jan 2025 21:50:55 +0800
From: Ethan Zhao <etzhao@...look.com>
To: Xin Li <xin@...or.com>, "H. Peter Anvin" <hpa@...or.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 dave.hansen@...ux.intel.com, tglx@...utronix.de, stable@...r.kernel.org,
 Ethan Zhao <haifeng.zhao@...ux.intel.com>
Subject: BUG ? exc_page_fault() was optimized out of fred_hwexc() by gcc with
 default kernel build option (-O2).

Hi, Xin, Peter

   While checking the asm code of arch/x86/entry/entry_fred.o about function fred_hwexc(),
found the code was generated as following :

0000000000000200 <fred_hwexc.constprop.0>:
  200:   0f b6 87 a4 00 00 00    movzbl 0xa4(%rdi),%eax
  207:   3c 0e                   cmp    $0xe,%al /* match X86_TRAP_PF */
  209:   75 05                   jne    210 <fred_hwexc.constprop.0+0x10>
  20b:   e9 00 00 00 00          jmp    210 <fred_hwexc.constprop.0+0x10>
  210:   3c 0b                   cmp    $0xb,%al
  212:   74 6a                   je     27e <fred_hwexc.constprop.0+0x7e>
  214:   77 17                   ja     22d <fred_hwexc.constprop.0+0x2d>
  216:   3c 06                   cmp    $0x6,%al
  218:   0f 84 83 00 00 00       je     2a1 <fred_hwexc.constprop.0+0xa1>
  21e:   76 29                   jbe    249 <fred_hwexc.constprop.0+0x49>
  220:   3c 08                   cmp    $0x8,%al
  222:   74 78                   je     29c <fred_hwexc.constprop.0+0x9c>
  224:   3c 0a                   cmp    $0xa,%al
  226:   75 18                   jne    240 <fred_hwexc.constprop.0+0x40>
  228:   e9 00 00 00 00          jmp    22d <fred_hwexc.constprop.0+0x2d>
  22d:   3c 11                   cmp    $0x11,%al
  22f:   74 66                   je     297 <fred_hwexc.constprop.0+0x97>
  231:   76 2c                   jbe    25f <fred_hwexc.constprop.0+0x5f>
  233:   3c 13                   cmp    $0x13,%al
  235:   74 5b                   je     292 <fred_hwexc.constprop.0+0x92>
  237:   3c 15                   cmp    $0x15,%al
  239:   75 1b                   jne    256 <fred_hwexc.constprop.0+0x56>
  23b:   e9 00 00 00 00          jmp    240 <fred_hwexc.constprop.0+0x40>
  240:   3c 07                   cmp    $0x7,%al
  242:   75 49                   jne    28d <fred_hwexc.constprop.0+0x8d>
  244:   e9 00 00 00 00          jmp    249 <fred_hwexc.constprop.0+0x49>
  249:   3c 01                   cmp    $0x1,%al
  24b:   74 3b                   je     288 <fred_hwexc.constprop.0+0x88>
  24d:   3c 05                   cmp    $0x5,%al
  24f:   75 1b                   jne    26c <fred_hwexc.constprop.0+0x6c>
  251:   e9 00 00 00 00          jmp    256 <fred_hwexc.constprop.0+0x56>
  256:   3c 12                   cmp    $0x12,%al
  258:   75 33                   jne    28d <fred_hwexc.constprop.0+0x8d>
  25a:   e9 00 00 00 00          jmp    25f <fred_hwexc.constprop.0+0x5f>

seems the following calling to exc_page_fault() was optimized out from fred_hwexc() by gcc,

if(likely(regs->fred_ss.vector==X86_TRAP_PF))
returnexc_page_fault(regs,error_code);

gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)

GNU objdump (GNU Binutils) 2.43


default kernel config.
.config:CONFIG_X86_FRED=y

my understanding, -O2 is the default kernel KBUILD_CFLAGS
So, Are there any workaround needed to make the kernel works with default build ?
or just as Peter said in another loop, manually loading some event bits to make the
over-smart gcc behave normally ？or fall back to -O(ption)0 ?

Any idea, much appreciated !


Thanks,
Ethan