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

[Nikunj A Dadhania]

Richard Henderson <address@hidden> writes:

> 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?

I did a quick test on power8, and it doesn't read back -1 after a write(-1).

int main(void)
{
  long res;
  asm volatile ("mtxer %1; mfxer %0;" : "=r"(res) : "r"(-1));
  printf("xer %lx\n", res);
}

$ ./a.out 
xer e00fffff
$

> 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.

Regards
Nikunj