[Bug report] Some mandatory CSRs are not implemented

Hi there!

I think I found a bug in Picorv32. Please let me know if it is incorrect.

Brief bug description

Some CSRs seem mandatory according to the specification, for example mvendorid.

The mvendorid CSR is a 32-bit read-only register providing the JEDEC manufacturer ID of the provider of the core. This register must be readable in any implementation, but a value of 0 can be returned to indicate the field is not implemented or that this is a non-commercial implementation.

However, reading such a register fails (more precisely, it hangs, like in #234). From my understanding of the code, the only CSRs supported by Picorv32 are some rvfi registers. I did not find documentation about CSRs in picorv32, maybe I missed something.

Tested parameters

I used the following parameters:

.ENABLE_COUNTERS      (1),
.ENABLE_COUNTERS64    (1),
.ENABLE_REGS_16_31    (1),
.ENABLE_REGS_DUALPORT (1),
.LATCHED_MEM_RDATA    (0),
.TWO_STAGE_SHIFT      (1),
.BARREL_SHIFTER       (0),
.TWO_CYCLE_COMPARE    (0),
.TWO_CYCLE_ALU        (0),
.COMPRESSED_ISA       (1),
.CATCH_MISALIGN       (1),
.CATCH_ILLINSN        (1),
.ENABLE_PCPI          (0),
.ENABLE_MUL           (1),
.ENABLE_FAST_MUL      (0),
.ENABLE_DIV           (1),
.ENABLE_IRQ           (0),
.ENABLE_IRQ_QREGS     (1),
.ENABLE_IRQ_TIMER     (1),
.ENABLE_TRACE         (0),
.REGS_INIT_ZERO       (0),
.MASKED_IRQ           (32'h 0000_0000),
.LATCHED_IRQ          (32'h ffff_ffff),
.PROGADDR_RESET       (32'h 8000_0000),
.PROGADDR_IRQ         (32'h 0000_0010),
.STACKADDR            (32'h ffff_ffff)

Example snippet

Here is an example snippet, given that, as specified by the RISC-V specification:

In systems without S-mode, the medeleg and mideleg registers should not exist.

The program below hangs after fetching the first nop.

  .section ".text.init","ax",@progbits
  .globl _start
  .align 2
_start:

  csrrw zero, mvendorid, zero

  nop
  nop

infinite_loop1:
  j infinite_loop1

Thanks! Flavien

YosysHQ / picorv32