[GitHub] [incubator-nuttx] no1wudi removed a comment on pull request #5115: arch/risc-v: Refine syscall interface

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no1wudi removed a comment on pull request #5115:
URL: https://github.com/apache/incubator-nuttx/pull/5115#issuecomment-1002954211


   @masayuki2009 @xiaoxiang781216 
   Hi, I find that this crash is caused by RISC-V's C extension , in riscv_syscall.S:
   ```
   sys_call0:	/* a0 holds the syscall number */		
    sys_call1:	/* a0 holds the syscall number, argument in a1 */		
    sys_call2:	/* a0 holds the syscall number, arguments in a1 and a2 */		
    sys_call3:	/* a0 holds the syscall number, arguments in a1, a2, and a3 */		
    sys_call4:	/* a0 holds the syscall number, arguments in a1, a2, a3 and a4 */		
    sys_call5:	/* a0 holds the syscall number, arguments in a1, a2, a3, a4 and a5 */		
   
     	/* Issue the ECALL opcode to perform a SW interrupt to the OS */		
   
     	ecall		
   
     	/* The actual interrupt may not a occur for a few more cycles.  Let's		
    	 * put a few nop's here in hope that the SW interrupt occurs during		
    	 * the sequence of nops.		
    	 */		
   
     	nop		
    	nop		
   
     	/* Then return with the result of the software interrupt in v0 */		
   
     	ret		
    	nop		
   ```
   All instruction use the normal 32bit instruction, but on some platform with C (Compressed instruction) support it would be 16bit, for FE310, you can adjust the mepc with 2 instead of 4:
   ```
   void *fe310_dispatch_irq(uint32_t vector, uint32_t *regs)
   {
     uint32_t  irq = (vector >> 27) | (vector & 0xf);
     uint32_t *mepc = regs;
   
     /* Firstly, check if the irq is machine external interrupt */
   
     if (FE310_IRQ_MEXT == irq)
       {
         uint32_t val = getreg32(FE310_PLIC_CLAIM);
   
         /* Add the value to nuttx irq which is offset to the mext */
   
         irq += val;
       }
   
     /* NOTE: In case of ecall, we need to adjust mepc in the context */
   
     if (FE310_IRQ_ECALLM == irq)
       {
         /* It should be 2 instead of 4 on target with C extension */
         *mepc += 4;
       }
    ...
   }
   ```
   @masayuki2009 Could you please have test ?
   


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