Re: [PATCH v4 11/11] KVM: selftests: Enable tunning of err_margin_us in arch timer test

[Marc Zyngier]

On 2023-12-20 06:50, Haibo Xu wrote:
> On Wed, Dec 20, 2023 at 2:22 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > On Tue, 12 Dec 2023 09:31:20 +0000,
> > Haibo Xu <haibo1.xu@xxxxxxxxx> wrote:
> > > > @@ -216,6 +221,9 @@ static bool parse_args(int argc, char *argv[])
> > >               case 'm':
> > >                       test_args.migration_freq_ms = atoi_non_negative("Frequency", optarg);
> > >                       break;
> > > +             case 'e':
> > > +                     test_args.timer_err_margin_us = atoi_non_negative("Error Margin", optarg);
> > > +                     break;
> >
> > So your error margin is always unsigned...
>
> The error margin was supposed to be a non-negative [0, INT_MAX].
> (May be need to define a Max for the input, instead of INT_MAX)
>
> > >               case 'o':
> > >                       test_args.counter_offset = strtol(optarg, NULL, 0);
> > >                       test_args.reserved = 0;
> > > diff --git a/tools/testing/selftests/kvm/include/timer_test.h b/tools/testing/selftests/kvm/include/timer_test.h
> > > index 968257b893a7..b1d405e7157d 100644
> > > --- a/tools/testing/selftests/kvm/include/timer_test.h
> > > +++ b/tools/testing/selftests/kvm/include/timer_test.h
> > > @@ -22,6 +22,7 @@ struct test_args {
> > >       int nr_iter;
> > >       int timer_period_ms;
> > >       int migration_freq_ms;
> > > +     int timer_err_margin_us;
> >
> > ... except that you are storing it as a signed value. Some consistency
> > wouldn't hurt, really, and would avoid issues when passing large
> > values.
>
> Yes, it's more proper to use an unsigned int for the non-negative error 
> margin.
> Storing as signed here is just to keep the type consistent with that
> of timer_period_ms
> since there will be '+' operation in other places.
>
>         tools/testing/selftests/kvm/aarch64/arch_timer.c
>         /* Setup a timeout for the interrupt to arrive */
>          udelay(msecs_to_usecs(test_args.timer_period_ms) +
>              test_args.timer_err_margin_us);

But that's exactly why using a signed quantity is wrong.
What does it mean to have a huge *negative* margin?

I don't see how you can justify this.

        M.
--
Jazz is not dead. It just smells funny...