Re: [PATCH v2 1/2] linux-user/s390x: signal with SIGFPE on compare-and-trap

[Richard Henderson <richard.henderson@linaro.org>]

On 7/7/21 6:42 AM, Jonathan Albrecht wrote:
> +                sig = TARGET_SIGFPE;
> +                if ((n & 0x03) == 0) {
> +                    /* An IEEE exception, simulated or otherwise. */
>                       if (n & 0x80) {
>                           n = TARGET_FPE_FLTINV;
>                       } else if (n & 0x40) {
> @@ -121,13 +123,12 @@ void cpu_loop(CPUS390XState *env)
>                           n = TARGET_FPE_FLTUND;
>                       } else if (n & 0x08) {
>                           n = TARGET_FPE_FLTRES;
> -                    } else {
> -                        /* ??? Quantum exception; BFP, DFP error.  */
> -                        goto do_sigill_opn;
>                       }
> -                    sig = TARGET_SIGFPE;
> -                    goto do_signal_pc;
> +                } else {
> +                    /* compare-and-trap */
> +                    n = 0;
>                   }

Nearly, but not quite.  Replace the ??? Quantum exception with n = 0, otherwise si_code 
will be garbage for that case.

The structure of the kernel code is

   if (n != 0) {
     /* do_fp_trap */
     si_code = 0;
     if ((n & 3) == 0) {
       /* select on bits 6 & 7 */
     }
     raise sigfpe w/ si_code
   } else {
     raise sigill
   }

The comment for compare-and-trap is misleading, because there are lots of entries in 
"Figure 6-2. Data-exception codes (DXC)" which arrive there and are not compare-and-trap.

As a general comment, I think a single switch over DXC would be cleaner for both kernel 
and qemu.  It seems like giving different si_code for e.g. "0x40 IEEE division by zero" 
and "0x43 Simulated IEEE division by zero" is actively incorrect.


r~