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

Marc Zyngier
2023-12-20T13:58:06Z

On Wed, 20 Dec 2023 13:51:24 +0000,
Haibo Xu <xiaobo55x@gmail.com> wrote:
>
> On Wed, Dec 20, 2023 at 5:00 PM Marc Zyngier <maz@kernel.org> wrote:
> >
> > On 2023-12-20 06:50, Haibo Xu wrote:
> > > On Wed, Dec 20, 2023 at 2:22 AM Marc Zyngier <maz@kernel.org> wrote:
> > >>
> > >> On Tue, 12 Dec 2023 09:31:20 +0000,
> > >> Haibo Xu <haibo1.xu@intel.com> wrote:
> > >> > 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?
> >
>
> Hi Marc,
>
> I agree that negative values are meaningless for the margin.
> If I understand correctly, the negative margin should be filtered by
> assertion in atoi_non_negative().

No. Please.

atoi_non_negative() returns a uint32_t, which is what it should do.
The bug is squarely in the use of an 'int' to store such value, and it
is the *storage* that turns a positive value into a negative one.

M.

--
Without deviation from the norm, progress is not possible.