On 1/18/2025 5:50 AM, Ethan Zhao wrote:
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 !
This is an optimization done in the original code:
static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code) { /* Optimize for #PF. That's the only exception which matters performance wise */ if (likely(regs->fred_ss.vector == X86_TRAP_PF)) return exc_page_fault(regs, error_code);
switch (regs->fred_ss.vector) {