Re: [Qemu-devel] [PATCH v2 9/9] target-arm: Add WFx instruction trap support

[Peter Maydell]

On 23 April 2015 at 03:49, Edgar E. Iglesias <address@hidden> wrote:
> On Wed, Apr 22, 2015 at 12:09:21PM -0500, Greg Bellows wrote:
>>  void HELPER(wfe)(CPUARMState *env)
>>  {
>>      CPUState *cs = CPU(arm_env_get_cpu(env));
>> +    int target_el = check_wfx_trap(env, true);
>>
>>      /* Don't actually halt the CPU, just yield back to top
>>       * level loop
>>       */
>> -    cs->exception_index = EXCP_YIELD;
>> +    if (target_el) {
>> +        cs->exception_index = EXCP_UDEF;
>> +        env->exception.syndrome = syn_wfx(1, 0xe, true);
>> +        env->exception.target_el = target_el;
>> +        env->pc -= 4;
>> +    } else {
>> +        cs->exception_index = EXCP_YIELD;
>> +    }
>>      cpu_loop_exit(cs);
>>  }
>
>
> Hi Greg,
>
> Before trapping, don't you need to check that the CPU has no actual work?
> e.g:
> if (!cc->has_work(cs) && ..)

We don't track WFE-wakeup events, so we must always trap. (Well,
I suppose strictly we could also go for "never trap" on the basis
that really our implementation here is pretty much a NOP, but I
think always-trap is closer to reasonable.)

In theory you could maybe check has_work() for the WFI case,
since doing an EXCP_HLT really should cause us to stop until
has_work is true, but it seems a bit fragile -- could we really
guarantee that nothing would change between this point and
when we went back through the main loop that would change
whether has_work evaluates true or not? I think that it's better
there too to just always take the trap: setting EXCP_HLT is our
"going into a low power state" and so we should take the trap
if we would otherwise have done that.

> I think you can still EXCP_YIELD the core if it has work though
> as it's probably a good place to reschedule other CPUs in our
> single-threaded SMP model.

That is indeed the only reason we do anything with WFE at all
(it used to be a no-op, but yielding gives better performance).

-- PMM