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

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Message-ID: <d9d943c7-e20c-4987-9c9e-076faad52558@linux.intel.com>
Date: Sun, 19 Jan 2025 17:43:34 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Ethan Zhao <etzhao@...look.com>, 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
Subject: Re: BUG ? exc_page_fault() was optimized out of fred_hwexc() by gcc
 with default kernel build option (-O2).

在 2025/1/18 21:50, Ethan Zhao 写道:
> 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
>
>

Rebuild the same lastest stable kernel with FRED enabled, default -O2 on RHEL 8.6, got
correct result:

ffffffff820a7430 <fred_hwexc>:
ffffffff820a7430:       55                      push   %rbp
ffffffff820a7431:       0f b6 87 a4 00 00 00    movzbl 0xa4(%rdi),%eax
ffffffff820a7438:       48 89 e5                mov    %rsp,%rbp
ffffffff820a743b:       3c 0e                   cmp    $0xe,%al
ffffffff820a743d:       75 0b                   jne    ffffffff820a744a <fred_hwexc+0x1a>
ffffffff820a743f:       e8 4c 50 00 00          callq  ffffffff820ac490 <exc_page_fault>
ffffffff820a7444:       5d                      pop    %rbp

gcc version 8.5.0 20210514 (Red Hat 8.5.0-10) (GCC)

whatever CONFIG_JUMP_LABEL=y or CONFIG_JUMP_LABEL is not set.

so the issue seems only about WSL ubuntu toolchain.

Thanks,
Ethan



>
>
-- 
"firm, enduring, strong, and long-lived"