Syscall Help

HiFive1 Rev B
1

I’ve got a user application running in u-mode with it’s stack frame protected by PMP.

PMP permissions:
pmp0cfg: 0x2004_0000 TOR RWX  //These values are before the bit shift 
pmp1cfg: 0x8000_2EE0 TOR RW
pmp2cfg: 0x8000_36E0 TOR RW

In user mode I call a syscall function (Rust)

let p1: usize = 22;
let p2: usize = 44;
unsafe{syscall(1,p1, p2)}

The only thing in the body of syscall() is inline assembly ecall
I figure the function call would put 1 into a0, p1 into a1, and p2 into a2 register where I could retrieve them from the trap_frame (where registers are saved on trap) to determine the syscall type and have a couple of parameters to use inside my trap handler.

However when I call my syscall function in u mode I get IllegalInstruction and the values of a0,a1,and a2 are all 0.

MTVAL = 0x30200073

mtval does not make sense to me, any ideas?

2

Oh, the mtval is the instruction itself for IllegalInstruction exception and is the address for address exceptions. The 0x30200073 makes more sense now

Machine Code
imm[11:0]    rs1  fn3   rd   opcode   
00110000001_00000_000_00000_1110011

I think that is right, but I don’t see 1110011 opcode in the base instruction set table? Plus
The source and destination registers don’t make sense either…hmm

3

Hi DK,

That opcode (0x30200073) is for MRET which can only be executed in machine mode. Could you be executing it from user mode?

We have a small syscall example here that might help:

4

Hey Ralph, thanks for the clarification. I don’t call mret until the end of my trap handler (if user ecall) which it is in m-mode at that point. I will check out the example code and see if I can spot my error. Thanks
Daniel

From that example, it looks like all it is doing is an ecall. I can simply ecall without issue, I’m trying to pass parameters along with the ecall in the a0…a[n] registers, that is what causes the problem.

5

Just wanted to come back and update this thread. This has been solved and was not a risc-v issue. I had a double dependency on a rust crate where the dependencies were of different versions. Once this was fixed, everything works as expected.