Re: [Qemu-devel] [PATCH v2 03/11] target/ppc: support for 32-bit carry and overflow

[Richard Henderson <rth@twiddle.net>]

On 02/22/2017 08:29 PM, Nikunj A Dadhania wrote:
>  target_ulong cpu_read_xer(CPUPPCState *env)
>  {
> -    return env->xer | (env->so << XER_SO) | (env->ov << XER_OV) |
> +    target_ulong xer;
> +
> +    xer = env->xer | (env->so << XER_SO) | (env->ov << XER_OV) |
>          (env->ca << XER_CA);
> +
> +    if (is_isa300(env)) {
> +        xer |= (env->ov32 << XER_OV32) | (env->ca32 << XER_CA32);
> +    }
> +    return xer;
>  }
>
>  void cpu_write_xer(CPUPPCState *env, target_ulong xer)
> @@ -32,5 +39,13 @@ void cpu_write_xer(CPUPPCState *env, target_ulong xer)
>      env->so = (xer >> XER_SO) & 1;
>      env->ov = (xer >> XER_OV) & 1;
>      env->ca = (xer >> XER_CA) & 1;
> -    env->xer = xer & ~((1u << XER_SO) | (1u << XER_OV) | (1u << XER_CA));
> +    if (is_isa300(env)) {
> +        env->ov32 = (xer >> XER_OV32) & 1;
> +        env->ca32 = (xer >> XER_CA32) & 1;
> +        env->xer = xer & ~((1ul << XER_SO) |
> +                           (1ul << XER_OV) | (1ul << XER_CA) |
> +                           (1ul << XER_OV32) | (1ul << XER_CA32));
> +    } else {
> +        env->xer = xer & ~((1u << XER_SO) | (1u << XER_OV) | (1u << XER_CA));
> +    }
>  }

Do cpus before power9 really save all of the bits you write to it?
I.e. if you write -1 to XER, do you read -1 back?

If so, then I suppose I'll have to revise my previous advice; there's no point 
in NOT storing bit 19 in env->ov32, because we can always read it back out.


r~