Re: [PATCH 09/10] target/arm: Implement FPSCR.LTPSIZE for M-profile LOB extension

[Richard Henderson]

On 10/12/20 8:37 AM, Peter Maydell wrote:
> @@ -198,8 +200,14 @@ void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t 
> val)
>          /*
>           * M profile FPSCR is RES0 for the QC, STRIDE, FZ16, LEN bits
>           * and also for the trapped-exception-handling bits IxE.
> +         * From v8.1M with the low-overhead-loop extension bits
> +         * [18:16] are used for LTPSIZE and (since we don't implement
> +         * MVE) always read as 4 and ignore writes.
>           */
>          val &= 0xf7c0009f;
> +        if (cpu_isar_feature(aa32_lob, cpu)) {
> +            val |= 4 << 16;
> +        }
>      }
>  
>      vfp_set_fpscr_to_host(env, val);
> @@ -212,9 +220,18 @@ void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t 
> val)
>       * (which are stored in fp_status), and the other RES0 bits
>       * in between, then we clear all of the low 16 bits.
>       */
> -    env->vfp.xregs[ARM_VFP_FPSCR] = val & 0xf7c80000;
> -    env->vfp.vec_len = (val >> 16) & 7;
> -    env->vfp.vec_stride = (val >> 20) & 3;
> +    if (cpu_isar_feature(aa32_lob, cpu)) {
> +        /*
> +         * M-profile low-overhead-loop extension: [18:16] are LTPSIZE
> +         * and we keep them in vfp.xregs[].
> +         */
> +        env->vfp.xregs[ARM_VFP_FPSCR] = val & 0xf7cf0000;
> +    } else {
> +        /* Those bits might be the old-style short vector length/stride */
> +        env->vfp.xregs[ARM_VFP_FPSCR] = val & 0xf7c80000;
> +        env->vfp.vec_len = (val >> 16) & 7;
> +        env->vfp.vec_stride = (val >> 20) & 3;
> +    }

I think these two sets of masking are confusing.
Perhaps usefully rearranged as

    if (!fp16) {
        val &= ~fz16;
    }
    vfp_set_fpscr_to_host(env, val);

    if (!m-profile) {
        vec_len = extract32(val, 16, 3);
        vec_stride = extract32(val, 20, 2);
    }
    val &= 0xf7c80000;
    if (lob) {
        val |= 4 << 16;
    }
    fpscr = val;

Which then obviates the next patch.


r~