Linux軟中斷與系統呼叫
阿新 • • 發佈：2018-12-25
1. SWI軟中斷
以ARMV7 A/R架構為例， SWI軟中斷和中斷一樣，核心空間處理始於異常向量表。Linux向量表預設地址0XFFFF0000，SWI向量偏移8位元組為0xFFFF0008：
具體程式碼，位於 \linux-3.4.x\arch\arm\kernel\entry-armv.S:
__vectors_start: ARM( swi SYS_ERROR0 ) THUMB( svc #0 ) THUMB( nop ) W(b) vector_und+ stubs_offset W(ldr) pc, .LCvswi + stubs_offset W(b) vector_pabt + stubs_offset W(b) vector_dabt + stubs_offset W(b) vector_addrexcptn + stubs_offset W(b) vector_irq + stubs_offset W(b) vector_fiq + stubs_offset .globl __vectors_end __vectors_end: .LCvswi: .word vector_swi
vector_swi的具體實現，位於 linux_plat\base\linux\linux-3.4.x\arch\arm\kernel\entry-common.S:
1 ENTRY(vector_swi) 2 sub sp, sp, #S_FRAME_SIZE 3 stmia sp, {r0 - r12} @ Calling r0 - r12 4 ARM( add r8, sp, #S_PC ) 5 ARM( stmdb r8, {sp, lr}^ ) @ Calling sp, lr6 THUMB( mov r8, sp ) 7 THUMB( store_user_sp_lr r8, r10, S_SP ) @ calling sp, lr 8 mrs r8, spsr @ called from non-FIQ mode, so ok. 9 str lr, [sp, #S_PC] @ Save calling PC 10 str r8, [sp, #S_PSR] @ Save CPSR 11 str r0, [sp, #S_OLD_R0] @ Save OLD_R0 12 zero_fp 13 14 /* 15 * Get the system call number. 16 */ 17 18 #if defined(CONFIG_OABI_COMPAT) 19 20 /* 21 * If we have CONFIG_OABI_COMPAT then we need to look at the swi 22 * value to determine if it is an EABI or an old ABI call. 23 */ 24 #ifdef CONFIG_ARM_THUMB 25 tst r8, #PSR_T_BIT 26 movne r10, #0 @ no thumb OABI emulation 27 ldreq r10, [lr, #-4] @ get SWI instruction 28 #else 29 ldr r10, [lr, #-4] @ get SWI instruction 30 A710( and ip, r10, #0x0f000000 @ check for SWI ) 31 A710( teq ip, #0x0f000000 ) 32 A710( bne .Larm710bug ) 33 #endif 34 #ifdef CONFIG_CPU_ENDIAN_BE8 35 rev r10, r10 @ little endian instruction 36 #endif 37 38 #elif defined(CONFIG_AEABI) 39 40 /* 41 * Pure EABI user space always put syscall number into scno (r7). 42 */ 43 A710( ldr ip, [lr, #-4] @ get SWI instruction ) 44 A710( and ip, ip, #0x0f000000 @ check for SWI ) 45 A710( teq ip, #0x0f000000 ) 46 A710( bne .Larm710bug ) 47 48 #elif defined(CONFIG_ARM_THUMB) 49 50 /* Legacy ABI only, possibly thumb mode. */ 51 tst r8, #PSR_T_BIT @ this is SPSR from save_user_regs 52 addne scno, r7, #__NR_SYSCALL_BASE @ put OS number in 53 ldreq scno, [lr, #-4] 54 55 #else 56 57 /* Legacy ABI only. */ 58 ldr scno, [lr, #-4] @ get SWI instruction 59 A710( and ip, scno, #0x0f000000 @ check for SWI ) 60 A710( teq ip, #0x0f000000 ) 61 A710( bne .Larm710bug ) 62 63 #endif 64 65 #ifdef CONFIG_ALIGNMENT_TRAP 66 ldr ip, __cr_alignment 67 ldr ip, [ip] 68 mcr p15, 0, ip, c1, c0 @ update control register 69 #endif 70 enable_irq 71 72 get_thread_info tsk 73 adr tbl, sys_call_table @ load syscall table pointer 74 75 #if defined(CONFIG_OABI_COMPAT) 76 /* 77 * If the swi argument is zero, this is an EABI call and we do nothing. 78 * 79 * If this is an old ABI call, get the syscall number into scno and 80 * get the old ABI syscall table address. 81 */ 82 bics r10, r10, #0xff000000 83 eorne scno, r10, #__NR_OABI_SYSCALL_BASE 84 ldrne tbl, =sys_oabi_call_table 85 #elif !defined(CONFIG_AEABI) 86 bic scno, scno, #0xff000000 @ mask off SWI op-code 87 eor scno, scno, #__NR_SYSCALL_BASE @ check OS number 88 #endif 89 90 ldr r10, [tsk, #TI_FLAGS] @ check for syscall tracing 91 stmdb sp!, {r4, r5} @ push fifth and sixth args 92 93 #ifdef CONFIG_SECCOMP 94 tst r10, #_TIF_SECCOMP 95 beq 1f 96 mov r0, scno 97 bl __secure_computing 98 add r0, sp, #S_R0 + S_OFF @ pointer to regs 99 ldmia r0, {r0 - r3} @ have to reload r0 - r3 100 1: 101 #endif 102 103 tst r10, #_TIF_SYSCALL_WORK @ are we tracing syscalls? 104 bne __sys_trace 105 106 cmp scno, #NR_syscalls @ check upper syscall limit 107 adr lr, BSYM(ret_fast_syscall) @ return address 108 ldrcc pc, [tbl, scno, lsl #2] @ call sys_* routine 109 110 add r1, sp, #S_OFF 111 2: mov why, #0 @ no longer a real syscall 112 cmp scno, #(__ARM_NR_BASE - __NR_SYSCALL_BASE) 113 eor r0, scno, #__NR_SYSCALL_BASE @ put OS number back 114 bcs arm_syscall 115 b sys_ni_syscall @ not private func 116 ENDPROC(vector_swi)
這個vector_swi函式完成的工作：
1. 保護異常前的現場
儲存現場大小為S_FRAME_SIZE，大小是一個完整的暫存器棧幀：
DEFINE(S_FRAME_SIZE, sizeof(struct pt_regs)); struct pt_regs { long uregs[18]; };
#define ARM_cpsr uregs[16]
#define ARM_pc uregs[15]
#define ARM_lr uregs[14]
#define ARM_sp uregs[13]
#define ARM_ip uregs[12]
#define ARM_fp uregs[11]
#define ARM_r10 uregs[10]
#define ARM_r9 uregs[9]
#define ARM_r8 uregs[8]
#define ARM_r7 uregs[7]
#define ARM_r6 uregs[6]
#define ARM_r5 uregs[5]
#define ARM_r4 uregs[4]
#define ARM_r3 uregs[3]
#define ARM_r2 uregs[2]
#define ARM_r1 uregs[1]
#define ARM_r0 uregs[0]
#define ARM_ORIG_r0 uregs[17]
從現場可以看出，每次使用者空間向核心空間切換時，執行緒核心棧會保留一份完整的暫存器現場，儲存地址在核心棧的（最高地址-8）：
執行緒核心棧基地址：0xc288a000
執行緒核心棧最高地址：0xc288a000+8k = 0xc288c000
儲存形式：
=============================低地址
R0 R1 R2 R3
R4 R5 R6 R7
R8 R9 R10 R11
R12 SP LR PC
CPSR R0_OLD X X
========================== kernel stack start(最高地址)
#define THREAD_SIZE 8192 #define THREAD_START_SP (THREAD_SIZE - 8)