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

[Peter Maydell <peter.maydell@linaro.org>]

On 23 April 2015 at 11:39, Edgar E. Iglesias <edgar.iglesias@gmail.com> wrote:
>
> On 23/04/2015 6:00 pm, "Peter Maydell" <peter.maydell@linaro.org> wrote:
>> 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.

Ah, so in helper_wfi() do something like

    if (!has_work()) {
       if (trapping wfi) {
           EXCP_UDEF code;
       } else {
           EXCP_HALT code;
       }
    }
    /* otherwise just return, making this WFI a nop */

?

I think that would work.

> Maybe wfe is the most problematic one because it fires more frequently and
> often when has_work() is true?

Yes, I think we should start by not trapping on WFE and then look
at how good/bad perf is.

-- PMM