linux-kernel - Re: [PATCH v4 11/11] KVM: selftests: Enable tunning of err_margin

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Date: Wed, 20 Dec 2023 21:51:24 +0800
From: Haibo Xu <xiaobo55x@...il.com>
To: Marc Zyngier <maz@...nel.org>
Cc: Haibo Xu <haibo1.xu@...el.com>, ajones@...tanamicro.com, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Paolo Bonzini <pbonzini@...hat.com>, 
	Shuah Khan <shuah@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	James Morse <james.morse@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Zenghui Yu <yuzenghui@...wei.com>, Anup Patel <anup@...infault.org>, 
	Atish Patra <atishp@...shpatra.org>, Guo Ren <guoren@...nel.org>, 
	Mayuresh Chitale <mchitale@...tanamicro.com>, Greentime Hu <greentime.hu@...ive.com>, 
	wchen <waylingii@...il.com>, Conor Dooley <conor.dooley@...rochip.com>, 
	Heiko Stuebner <heiko@...ech.de>, Minda Chen <minda.chen@...rfivetech.com>, 
	Samuel Holland <samuel@...lland.org>, Jisheng Zhang <jszhang@...nel.org>, 
	Sean Christopherson <seanjc@...gle.com>, Peter Xu <peterx@...hat.com>, Like Xu <likexu@...cent.com>, 
	Vipin Sharma <vipinsh@...gle.com>, 
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, Aaron Lewis <aaronlewis@...gle.com>, 
	Thomas Huth <thuth@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, kvm-riscv@...ts.infradead.org
Subject: Re: [PATCH v4 11/11] KVM: selftests: Enable tunning of err_margin_us
 in arch timer test

On Wed, Dec 20, 2023 at 5:00 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On 2023-12-20 06:50, Haibo Xu wrote:
> > On Wed, Dec 20, 2023 at 2:22 AM Marc Zyngier <maz@...nel.org> wrote:
> >>
> >> On Tue, 12 Dec 2023 09:31:20 +0000,
> >> Haibo Xu <haibo1.xu@...el.com> 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?
>

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().

Thanks,
Haibo

> I don't see how you can justify this.
>
>          M.
> --
> Jazz is not dead. It just smells funny...