OpenRISC

User Mode FPCSR (P17) 2020-05-22 - Stafford Horne

While working on the glibc port for OpenRISC we found that pretty much every architecture allows reading and writing to the floating point control register (in OpenRISC FPCSR) in userland. This is used by userland to control the rounding mode and exception reading and controlling of the FPU.

However, on OpenRISC this is only writable in supervisor mode. We can read it by enabling a flag in the SR; but not write to it.

I propose that we:

  1. Allow both read and write to FPCSR always

This will require changes to:

  • The architecture specification
    • Update table 4-2 to mention R/W for FPCSR
    • Update section 4.12 to mention “as a read/write register in user mode”
  • Linux Kernel preserve FPCSR during context switches
  • Qemu to allow writes to FPCSR (I have done part of this already, as its needed for glibc testing)
  • Other simulators.
  • Mor1kx and Marocchino verilog cores