Re: [PATCH] target/hppa: Set FPCR exception flag bits for non-trapped exceptions

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Cc'ing Gabriel

On 17/10/25 10:53, Peter Maydell wrote:
> In commit ebd394948de4e8 ("target/hppa: Fix FPE exceptions") when
> we added the code for setting up the registers correctly on trapping
> FP exceptions, we accidentally broke the handling of the flag bits
> for non-trapping exceptions.
> 
> In update_fr0_op() we incorrectly zero out the flag bits and the C
> bit, so any fp operation would clear previously set flag bits. We
> also stopped setting the flag bits when the fp operation raises
> an exception and the trap is not enabled.
> 
> Adjust the code so that we set the Flag bits for every exception that
> happened and where the trap is not enabled.  (This is the correct
> behaviour for the case where an instruction triggers two exceptions,
> one of which traps and one of which does not; that can only happen
> for inexact + underflow or inexact + overflow.)
> 
> Cc: qemu-stable@nongnu.org
> Fixes: ebd394948de4e8 ("target/hppa: Fix FPE exceptions")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3158
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   target/hppa/fpu_helper.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/target/hppa/fpu_helper.c b/target/hppa/fpu_helper.c
> index 45353202fae..2d272730f60 100644
> --- a/target/hppa/fpu_helper.c
> +++ b/target/hppa/fpu_helper.c
> @@ -94,7 +94,8 @@ static void update_fr0_op(CPUHPPAState *env, uintptr_t ra)
>   {
>       uint32_t soft_exp = get_float_exception_flags(&env->fp_status);
>       uint32_t hard_exp = 0;
> -    uint32_t shadow = env->fr0_shadow & 0x3ffffff;
> +    uint32_t shadow = env->fr0_shadow;
> +    uint32_t to_flag = 0;
>       uint32_t fr1 = 0;
>   
>       if (likely(soft_exp == 0)) {
> @@ -122,6 +123,10 @@ static void update_fr0_op(CPUHPPAState *env, uintptr_t ra)
>               fr1 |= hard_exp << (R_FPSR_FLAGS_SHIFT - R_FPSR_ENABLES_SHIFT);
>           }
>       }
> +    /* Set the Flag bits for every exception that was not enabled */
> +    to_flag = hard_exp & ~shadow;
> +    shadow |= to_flag << (R_FPSR_FLAGS_SHIFT - R_FPSR_ENABLES_SHIFT);
> +
>       env->fr0_shadow = shadow;
>       env->fr[0] = (uint64_t)shadow << 32 | fr1;
>