RISCV dynamic rounding mode is not behaving correctly

[Mina Magdy]

Hello,

I’ve gone through the RISC-V code in latest QEMU release (qemu-5.0.0-rc2) and when checking the Floating point encodings I found the rounding mode is only updated if the opcode field “rm” is changed “ctx->frm == rm”. But according to RISC-V Volume I: Unprivileged ISA, there’s a dynamic mode when rm=7 where the rounding mode is set with frm value.

So for the same rm value (=7) and when changing frm value seeking different rounding modes, and according to the below code, the rounding mode won’t be updated. Please correct me if I got this implementation wrong.

static void gen_set_rm(DisasContext *ctx, int rm)
{
    TCGv_i32 t0;
    if (ctx->frm == rm) {
        return;
    }
    ctx->frm = rm;
    t0 = tcg_const_i32(rm);
    gen_helper_set_rounding_mode(cpu_env, t0);
    tcg_temp_free_i32(t0);
}

My testcase:
I set statically the rm field in the instruction to 7 and before this execution I changed the value of frm field in fcsr register. For the 1st time it worked (according to the code above, the rm is updated so the round mode will also be updated). But when changing fcsr register an re-execute the instruction, there's no difference and the rounding mode is the same like the previous frm value.

[Mina Magdy (rpvrverve453-4tr5t34t5)]

After checking RISCY RTL code, i found the implementation is straight forward as stated in specs as follows:
              if (FPU == 1) begin
                 if (fp_rnd_mode == 3'b111)
                   apu_flags = frm_i;
                 else
                   apu_flags = fp_rnd_mode;
              end else

where fp_rnd_mode is the round mode field in the instruction opcode.

[Alistair Francis (alistair2323)]

This does look like incorrect behaviour. I have sent a patch to the mailing list. You can see the patch here: https://<email address hidden>/ea4f280e6f77e734c8e555e3c98d10085ce9f5b6<email address hidden>/

[Mina Magdy (rpvrverve453-4tr5t34t5)]

Thank you Alistair Francis.

[Alistair Francis (alistair2323)]

As commented on the patch submission, this should already be handled: https://<email address hidden>/msg718331.html

Can you attach the test case that is failing?

[Thomas Huth (th-huth)]

The QEMU project is currently moving its bug tracking to another system.
Is there still anything left to do here? If so, please provide the test case and switch the state back to "New" or "Confirmed", or open a new ticket in the new bug tracker here: https://gitlab.com/qemu-project/qemu/-/issues

[Launchpad Janitor (janitor)]

[Expired for QEMU because there has been no activity for 60 days.]