[BUG] CGetOffset and CSetEqualExact give wrong result

[Andreas-Bur]

Is there an existing CVA6 bug for this?

• I have searched the existing bug issues

Bug Description

I encountered the case where cgetoffset gives the wrong result when operating on a register previously written to by csrrw. Here a small PoC:
In the end register x4 has value 0x000fffffffffffff instead of 0xffffffffffffffff.

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

# Init FPU

  lui x11, 2
  csrrw x11, mstatus, x11
  addi x11, x0, 0
  csrrw x11, fcsr, x11

# Program start

  li x1, -1
  csrrw x2,sscratch,x1
  csrrw x3,sscratch,x2
  cgetoffset x4, c3
  cgetoffset x5, c1

# Dump capability registers

  cspecialrw c29, 29, c0
  fence
  csc c1, 48(c29) # Writes 0x0000000000010004ffffffffffffffff
  fence
  csc c2, 48(c29) # Writes 0x00000000000100040000000000000000
  fence
  csc c3, 48(c29) # Writes 0x0000000000010004ffffffffffffffff
  fence
  csc c4, 48(c29) # Writes 0x0000000000010004000fffffffffffff instead of 0x0000000000010004ffffffffffffffff
  fence
  csc c5, 48(c29) # Writes 0x0000000000010004ffffffffffffffff
  fence

[Andreas-Bur]

The same problem also occurs with CSetEqualExact:

# Init FPU

  lui	x11, 2
  csrw	mstatus, x11
# Program start

  li x1, 0xFFFFFFFFFFFFFFFF

  csrrw           x2,sscratch,x1
  csrrw           x2,sscratch,x1
  cseqx           x3,c2,c1  # Writes 0 to x3