kernel是如何调用sbi里面的函数

本文以OpenSBI 0.9版本为参考进行讲解

现在分析kernel是如何调用sbi里面的函数的

1. 在sbi的初始化过程中，在fw_base.S里面会设置M mode的异常处理函数：
   la a4, _trap_handler
   …
   csrw CSR_MTVEC, a4
   设置完成后，只要发生M mode的异常就会进入到_trap_handler里面处理，
   _trap_handler这个函数在fw_base.S里面有定义，是汇编代码，实现了异常/中断调用的各个流程，
   _trap_handler:
   TRAP_SAVE_AND_SETUP_SP_T0

    TRAP_SAVE_MEPC_MSTATUS 0
   
    TRAP_SAVE_GENERAL_REGS_EXCEPT_SP_T0
   
    TRAP_CALL_C_ROUTINE   --------------------------这里进入异常处理的c函数，也就是sbi_trap_handler
   
    TRAP_RESTORE_GENERAL_REGS_EXCEPT_SP_T0
   
    TRAP_RESTORE_MEPC_MSTATUS 0
   
    TRAP_RESTORE_SP_T0
   
    mret
   

   sbi_trap_handler函数实现在/lib/sbi/sbi_trap.c里面。
   发生异常/中断的时候会进入到sbi_trap_handler函数，根据 mcause = csr_read(CSR_MCAUSE);
   的值，进入不同的处理分支，kernel下发的ecall会进入到：
   case CAUSE_SUPERVISOR_ECALL:
   case CAUSE_MACHINE_ECALL:
   rc = sbi_ecall_handler(regs);
   这里面，接着调用 sbi_ecall_find_extension(extension_id);
   struct sbi_ecall_extension *sbi_ecall_find_extension(unsigned long extid)
   {
   struct sbi_ecall_extension *t, *ret = NULL;

    	sbi_list_for_each_entry(t, &ecall_exts_list, head) {
    		if (t->extid_start <= extid && extid <= t->extid_end) {
    			ret = t;
    			break;
    		}
    	}
   
    	return ret;
    }
   

   通过extid获取到具体extension的handler，所有的异常处理在sbi初始化阶段，调用sbi_ecall_init
   的时候，全部注册在了ecall_exts_list里面。这个时候会从ecall_exts_list里面找到
   对应的handler。

2. 以sbi_ecall_init()为例，介绍异常处理handler的处理过程
   2.1 注册了很多的异常回调handler
   int sbi_ecall_init(void)
   {
   int ret;

    	/* The order of below registrations is performance optimized */
    	ret = sbi_ecall_register_extension(&ecall_time);
    	if (ret)
    		return ret;
    	ret = sbi_ecall_register_extension(&ecall_rfence);
    	if (ret)
    		return ret;
    	ret = sbi_ecall_register_extension(&ecall_ipi);
    	if (ret)
    		return ret;
    	ret = sbi_ecall_register_extension(&ecall_base);
    	if (ret)
    		return ret;
    	ret = sbi_ecall_register_extension(&ecall_hsm);
    	if (ret)
    		return ret;
    	ret = sbi_ecall_register_extension(&ecall_srst);
    	if (ret)
    		return ret;
    	ret = sbi_ecall_register_extension(&ecall_legacy);
    	if (ret)
    		return ret;
    	ret = sbi_ecall_register_extension(&ecall_vendor);
    	if (ret)
    		return ret;
   
    	return 0;
    }
   

   2.2
   sbi_ecall_register_extension(&ecall_hsm);
   int sbi_ecall_register_extension(struct sbi_ecall_extension *ext)
   {
   struct sbi_ecall_extension *t;

    		if (!ext || (ext->extid_end < ext->extid_start) || !ext->handle)
    			return SBI_EINVAL;
   
    		sbi_list_for_each_entry(t, &ecall_exts_list, head) {
    			unsigned long start = t->extid_start;
    			unsigned long end = t->extid_end;
    			if (end < ext->extid_start || ext->extid_end < start)
    				/* no overlap */;
    			else
    				return SBI_EINVAL;
    		}
   
    		SBI_INIT_LIST_HEAD(&ext->head);
    		sbi_list_add_tail(&ext->head, &ecall_exts_list);
   
    		return 0;
    	}
    会把ecall_hsm放到ecall_exts_list链表里面，ecall_hsm里面有extid和具体的handler。
    struct sbi_ecall_extension ecall_hsm = {
    	.extid_start = SBI_EXT_HSM,
    	.extid_end = SBI_EXT_HSM,
    	.handle = sbi_ecall_hsm_handler,
    };
   

到这里，sbi中M mode的异常处理已经注册完成，这时候只要出发M mode的异常就会进入M mode，进入对应的handler。

3. kernel是如何出发M mode的异常的：
   3.1
   在arch/riscv/kernel/sbi.c里面有sbi_ecall()调用函数，kernel可以通过sbi_ecall来出发M mode的异常处理。
   struct sbiret sbi_ecall(int ext, int fid, unsigned long arg0,
   unsigned long arg1, unsigned long arg2,
   unsigned long arg3, unsigned long arg4,
   unsigned long arg5)
   {
   struct sbiret ret;

    	register uintptr_t a0 asm ("a0") = (uintptr_t)(arg0);
    	register uintptr_t a1 asm ("a1") = (uintptr_t)(arg1);
    	register uintptr_t a2 asm ("a2") = (uintptr_t)(arg2);
    	register uintptr_t a3 asm ("a3") = (uintptr_t)(arg3);
    	register uintptr_t a4 asm ("a4") = (uintptr_t)(arg4);
    	register uintptr_t a5 asm ("a5") = (uintptr_t)(arg5);
    	register uintptr_t a6 asm ("a6") = (uintptr_t)(fid);
    	register uintptr_t a7 asm ("a7") = (uintptr_t)(ext);
    	asm volatile ("ecall"
    			  : "+r" (a0), "+r" (a1)
    			  : "r" (a2), "r" (a3), "r" (a4), "r" (a5), "r" (a6), "r" (a7)
    			  : "memory");
    	ret.error = a0;
    	ret.value = a1;
   
    	return ret;
    }
   
    各种异常都是通过sbi_ecall来实现的，在sbi.c以及cpu_ops_sbi.c里面已经封装了各种的实现，
    如启动core的函数sbi_hsm_hart_start -> 
    sbi_ecall(SBI_EXT_HSM, SBI_EXT_HSM_HART_START, hartid, saddr, priv, 0, 0, 0);
    最终调用的就是sbi里面注册的ecall_hsm。