On 23/04/2015 6:00 pm, "Peter Maydell" <peter.mayd...@linaro.org> wrote:
>
> On 23 April 2015 at 03:49, Edgar E. Iglesias <edgar.igles...@gmail.com>
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 functional wise we are OK.
The implementation can AFAIK always choose to nop for whatever reason (e.g
has_work()). Only when we choose to enter low power, the trap comes into
play.
Maybe wfe is the most problematic one because it fires more frequently and
often when has_work() is true?
Cheers,
Edgar
>
> > 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
